Note: significant updates to Load Factor, Train ID, minor updates throughout. (09/26/2016)

See BART Service Advisory or BSA.
Application Programming Interface (API) is a set of routines, data structures, object classes and/or protocols provided by libraries and/or operating system services in order to support the building of applications. In the case of the BART API, it is a set of programming calls that return XML data about the BART service and system.
Associated Route
Each line is made up of two routes, one north and one south. For a given route, the "associated route" is the other route that makes up the complete line (e.g. Route 2's associated route is Route 1).
Available Schedules
A list of all of the current schedules that are included in the BART API. This may include schedules that are no longer active, a current schedule, and an upcoming schedule, depending on what has been released, and if a new schedule has been finalized. Checking the available schedules via the API will provide the schedule number and the effective date of that schedule for each of the schedules included in the BART schedule system. Old schedules are periodically removed, so requests for dates in the past will return results from the oldest available schedule.
BART Midnight
A special designation for a time between BART schedules. Since BART runs from approximately 4 am until just after 1 am, the concept of "BART Midnight" is used to represent a time that is between BART schedules. Current it is set to 2:26 am, but in the future it may vary depending on the day of week or other factors. For additional information about this please refer to the BART Time information.
The BART schedule use a special clock that is greater than 24 hours. It runs basically from midnight until 2am the following day. This has implications with certain QuickPlanner/Schedule results. For more information about this, please read the BART Time information.
Bike Flag
Designates if it is okay to bring bicycles on a specific train. This flag appears in the QuickPlanner and the station schedules. Bicycles are restricted from some trains during peak commute due to space limitations. bikeflag = 1, bikes allowed. bikeflag = 0, no bikes allowed. Note: BART still maintains this flag, but currently there are no bike restrictions in the current schedule. For more information, please refer to BART Bike Rules.
BART Service Advisory. Messages entered by the BART Operations Control Center (OCC) to let BART riders know about conditions that could affect the BART system. These include delay messages, warnings of police action, equipment problems, etc. Service advisories are only issued if two or more trains are off schedule by more than 10 minutes.
Bus Bridge
Sometimes when service between two stations is interrupted in such a way that trains cannot get through, connecting transit partners will be brought in to shuttle people between the two stations.
A text value for the line color of a particular train. Possible values are: BLUE, GREEN, ORANGE, RED, YELLOW, BEIGE and WHITE. (WHITE represents trains that were added and have not been given a specific line assignment yet and the BEIGE route does not currently have ETD data.)
Another word used for train within BART. The two terms are interchangeable.
CO2 Emissions
The approximate amount of Carbon Dioxide saved using BART versus driving a car. The QuickPlanner automatically calculates this based on a formula of how far the BART trip is and what the average per car uses. More information can be found on the BART website.
Dwell Time
The amount of time a train spends at a station before continuing along the route, or returning back on the associated route on the line.
Elevator Advisory
Message detailing any current elevator outages within the BART system.
Escalator Advisory
Message detailing any current escalator outages within the BART system. [Coming to API in 2017]
Estimated Time of Departure. Specifies time remaining before a train leaves a station. Since most dwell times are less than 1 minute at non-terminus stations, ETD's are equivalent to Estimated Time of Arrivals (ETA's). For trains with less than 1 minute left before departure, the ETD data will show the time as "Leaving". The BART website and API exclusively use ETD (not ETA) for all data.
Excursion Fare
A special price for trips that start and end at the same BART station. The current excursion fare can be found using this fare command
The cost of a trip between two stations on the BART system. Fares are calculated by planning a trip between the two stations using the same back-end processes that are used on the BART website. Trips starting from and ending at the same station will result in the Excursion fare. Fare information now includes a fare class (current all fares are "normal") and then various fares (cash, rtcclipper).
Generic Route Schedule
A schedule for a given type of day (weekday, Saturday or Sunday), but not tied to a specific date. There are several important caveats for using this information. Please see the details posted in the BART Developers forum.
A hexidecimal RGB color value for the line color of a particular train. These colors match the color scheme used on the BART website for the various lines. The Routes and Route Information commands return this in the <color> element. BART will work to make these consistent with the ETD data in future releases.
On certain specified days, the BART system may run a special schedule that differs from the normal daily schedule. These special schedules are automatically used when determining a trip schedule with the QuickPlanner. You can also find out what days, and types of schedules, BART will use for holidays using the Holiday API call.
Limited Trains
A trains that does not stop at all of the stations on a particular route. This may also be a short train. Limited trains will have " - Limited" after the name of the head station that is the actual terminus of the train, and are listed by that head station in results from the QuickPlanner and ETD displays. On the BART website homepage and Mobile site the limited trains are displayed with a " - L" after the station abbreviation for the terminus station.
An inclusive collection of stations between two endpoints. Lines do not have a direction, but are made up of two routes which are uni-directional inclusive paths between the two end stations. The API does not provide any information about lines, but only the component routes.
Load Factor
An estimate of how full the train is at a specified station. This is based on historical data, and is not an indication of real-time conditions. The load factor will be a positive number, if defined, or 0 if no load factor is currently available for the specified train, route and schedule type. The higher the number, the more crowded the train may be.
Currently only weekday schedules have load factor information available. Added trains (trains placed into service either to replace another train or to provide additional service) will not have an associated load factor.
Provides schedule information between BART stations for the specified date and time. The QuickPlanner in the API uses the same back-end processes that are used on the BART website, so trips planned using the QuickPlanner API will match exactly with the results from the website.
This designates a specific area within the station for boarding trains. All stations have at least 2 platforms and some have more. Generally a platform is associated with a particular direction of travel, but in the event of a disruption to regular service, a platform may be used for other trains as needed. Thus is it generally not wise to show specific platforms in trip planning, or at least to mention that the rider "Always check destination signs and listen for departure announcements".
A uni-directional inclusive collection of stations between two end stations. Each BART line has two routes associated with it, one for each direction. The lines are considered to run North and South. (Dublin/Pleasanton to Daly City is considered the South route for the DUBL-DALY line.) Routes have a name (Dublin/Pleasanton - Daly City), abbreviation (DUBL-DALY), routeID (ROUTE 11), number (11), color (#0099cc), holiday flag (0), and a number of stations (17) as well as other information.
Route Number
An identifier used to designate one of the two directions for a given line. Each line has two routes, each with its own number. The route number is a single number, while the routeID is the combination of the word "ROUTE" and the number. (e.g. the Millbrae/Daly City - Richmond route is routeID "ROUTE 8", and route number 8.)
Route Schedule
Provides a complete listing of all scheduled trips between all stations on a line in a specific direction (Northbound or Southbound). These are called using the route number
Service Advisory
See BART Service Advisory or BSA.
Short Train
A trains that does not run through all of the stations on a particular route. They will display a head station that is the actual terminus of the train, and will be designated by that head station in results from the QuickPlanner and station schedules.
Special Schedule
Occasionally BART has maintenance work or other situations that may affect certain trips. These messages provide information about which stations are affected and the dates and times when the special schedule might occur. These special schedule messages are automatically generated for any QuickPlanner trips created using the API, but can also be returned directly with an API call.
Station Abbreviations
Four letter codes that are used to uniquely identify a BART station. BART maintains a list of the station abbreviations.
Station Closure
In rare occasions, BART may shut down a station due to an emergency or due to safety concerns. At these times, riders will be directed via announcements and or signs as to what they will need to do. Often BART will set up bus bridges to ferry riders around a closure, or may run the trains through the affected station without stopping. In terms of the API, this might be reflected in a BSA or a special schedule announcement (if planned ahead).
Station Schedule
Provides all of the departure times for a specified station. Includes information on the route, the train head station, the departure time, the destination time at the terminus station, the train id, the train index, and the bike flag along with other station schedule information.
A station at the end of a route. Trains typically sit at terminus stations for a brief period before starting a return trip on the other route that makes up the specific line.
Three or more BART cars travelling along a route. The minimum size for a BART train is three cars, and the maximum is 10 cars. Trains are also sometimes called a consist. The term can be used interchangeably.
Train Count
The current number of active trains in the BART system.
Train Head Station
Each train has displays a destination station on the front, which is also used on the displays in the station, and for announcements made by the Train Operator This station is the final destination for the train on the current trip. It might differ from the route end station if the train is a short train. This is also referred to as the Train Headway.
Train Id
A unique identifier of a train within each schedule type. (e.g. These ids are unique to each Weekday/Saturday/Sunday schedules, but may be repeated across the different types.) This information can be used to match schedule and real-time information. Note: BART is still working on adding the Train ID information into the ETD real-time data.
Train Index
An identifier of a train along a specified route. These are shown in the QuickPlanner and Station Schedule results, and increment for each train that runs along a specific route. Each route has its own index, so there may be several trains with the same index on different routes. In addition, some routes have short trains, which will use the same sequence as the full train for that route. (e.g. at Embarcadero, some SFIA trains only run to MONT during rush hour. These trains will still be included in the index sequence for SFIA.)
Trip Time
The total minutes a trip will take according to the BART schedule.