| Abbreviation | Definition |
|---|---|
| AC | Active Coronae |
| AC | Ascendant Cusp |
| ACA | Atacama Compact Array |
| ACIT | Automatic Comet Imaging Telescope |
| ACME | Astronomy for Catholics in Ministry and Education |
| ACS | Advanced Camera for Surveys |
| ACT | Astrographic Catalog Tycho |
| ACTS | Advanced Communications Technology Satellite |
| ACURA | Association of Canadian Universities for Research in Astronomy |
| AD | Astronomsko Drustvo |
| ADACS | Astronomy Data and Compute Services |
| ADACS | Astronomy Data and Computing Services |
| ADAF | Advection-Dominated Accretion Flow |
| ADAS | Asiago DLR Asteroid Survey |
| ADASS | Astronomical Data Analysis Software and Systems |
| ADH | Average Daily Hours |
| ADIL | Astronomy Digital Image Library |
| ADS | Astrophysics Data System |
| ADU | Astronomical Distance Unit |
| AEA | Atomic Energy Agency |