| 11D | 11th Dimension | |
| 4D | 4th Dimension | |
| 5D | Fifth-Dimensional | |
| A-GPS | Augmented Global Positioning System | |
| A-GPS | Assisted Global Positioning System | |
| AAA | Amateur Astronomers Association | |
| AAAA | Asociación Argentina Amigos de la Astronomía | |
| AAAD | Amateur Astronomers Association of Delhi | |
| AAAD | Amateur Astronomers Association Delhi | |
| AAAM | Active Amateur Astronomers of Maryland | |
| AAG | Amateur Astronomy Group | |
| AAJ | Astronomical Association of Jamaica | |
| AAL | Astronomy Associates of Lawrence | |
| AAL | Astronomy Associates of Lawrence Astronomy Associates of Lawrence | |
| AAPF | Astronomy and Astrophysics Postdoctoral Fellowships | |
| AAQ | Astronomical Association of Qld | |
| AAS | American Astronautical Society | |
| AAS | Archaeology, Astronautics, and Seti | |
| AASC | Astronomy and Astrophysics Survey Committee | |
| AASI | Astronomical Association of Southern Illinois | |
| AAT | Anglo Australian Telescope | |
| AAV | Astronomical Analogue Video | |
| AAVSO | American Association of Variable Star Observers | |
| ABB | After the Big Bang | |
| ABBA | ADC Backend For Bolometer Array | |
