APRS - Automatic Packet Reporting System, invented by Bob Bruninga, WB4APR.
APRS-IS - The APRS Internet System, a backbone for Internet relaying of APRS packets through IGate stations (routing between the Internet and local RF networks).
AVRS - Automatic Voice Response System - a service implemented over APRS to aid in linking amateur radio voice communications over Echolink, IRLP, or other Internet-to-radio relaying protocols.
AX.25 - OSI Level 2 protocol used for amateur radio digital packet communications. It supports both a reliable connected-mode session-oriented protocol (I frames) and an unreliable broadcast mode (UI frames).
beacon - transmissions by APRS stations reporting their current position and status to provide current tactical information to listening stations.
CC-BY-SA - Creative Commons license, the old license under which OpenStreetMap data is made available, replaced by the Open Database License.
digipeater - AX.25 station which will rebroadcast received data packets to extend their range, generally requested by the transmitting station specifying known standard digipeater aliases (such as WIDE1-1).
Echolink - a software-based protocol for relaying amateur radio voice communications over the Internet.
GPS - Global Positioning System, generally referring to the satellite navigation system operated by the United States Department of Defense. There are other navigation systems, such as GLONASS, which generically fall into the category of GPS, but are not the U.S. DoD system.
I-Gate - an AX.25 station which repeats APRS packets between RF and the APRS-IS Internet backbone; I-Gates can repeat RF-to-Internet, Internet-to-RF, or both ways. See the tutorial for more information.
IRLP - Internet Relay Link Protocol, a communications standard for dynamically linking and unlinking RF repeater stations over the Internet.
Mic-E - a compressed binary position message format used by several transceivers with built-in APRS capability and dedicated tracker devices.
NMEA - National Marine Electronics Association, standards body for marine electronic equipment, including NMEA-0183, the GPS sentence protocol standard.
OpenStreetMap - an open-source project for collecting street map data, hosted on the Internet at https://www.openstreetmap.org.
OpenTRAC - the Open Tactical Reporting And Communications protocol, a proposed binary protocol for messaging position and status information between stations, as a newer alternative to APRS. YAAC supports transmitting and receiving OpenTRAC messages over SerialTNC and AGWPE ports (RF connectivity); the APRS-IS network does not support OpenTRAC. Further information on OpenTRAC available at the OpenTRAC website.
SRTM - Shuttle Radar Topographic Mission, a series of Space Shuttle missions to map the physical geography of the world; the data collected by these missions is available from the U.S. Geological Survey at https://e4ftl01.cr.usgs.gov and may also be downloaded through YAAC.
tactical callsign - a callsign describing the function of a particular station, rather than the government-assigned identifier of the station licensee. Generally useful only in a local operation or under a single Incident Command System. Can be displayed in YAAC by defining local aliases for station callsigns.
TNC - Terminal-Node Controller, the modem and packet assembler/disassembler (PAD) used to package and unpackage AX.25 data frames to and from radios.
UTM - Universal Transverse Mercator, a map projection based on the Mercator projection with adaptations to allow reasonably accurate great-circle distance tracing between points on the map with known limited errors, regardless of earth curvature issues. Often used by the military (MGRS system) and by hikers and search-and-rescue teams.
vicinity plotting - a technique used by APRS mapping clients (including YAAC) to approximate the location of a station that has transmitted traffic but not position messages, by guesstimating the station's position as near the first digipeating station to hear the unknown station.
WGS84 - World Geodetic Standard 1984, the reference datum for coordinate systems used by the Global Positioning System, OpenStreetMap, and YAAC.
Winlink 2000 - the radio-based e-mail system that is accessible from APRS via the APRSlink gateway (callsign WLNK-1).
YAAC - Yet Another APRS Client (not to be confused with Yet Another Compiler Compiler or Yet Another Audio Codec).