| Acronym | Meaning |
| (PI)SDCI | (pi)-electron Singly And Doubly Excited CI |
| (SC)2(CI+PT2) | Self-consistent Size-consistent (CI+second-order Perturbation) |
| (SC)2CI | Self-consistent Size-consistent CI |
| 2DEG | Two-dimensional Electron Gas |
| 2DHG | Two Dimensional Hole Gas |
| 2MASS | Two Micron All-Sky Survey |
| 2PH TDA | Two-particle-hole Tamm-Dancoff Approximation (for Closed-shell |
| 2PPE | Two-photon Photoemission |
| 3A | Alumina Aluminide Alloys |
| 3DAP | Three Dimensional Atom Probe |
| 3DWQM | Chesapeake Bay Three Dimensional Time Variable Water Quality Model |
| 3P | Pollution Prevention Pays |
| 3SR | Self-sustained Sequence Replication |
| 4WM | Four Wave Mixing |
| 6FDA | 2,2'-Bis (3,4-Dicarboxyphenyl) Hexafluoropropane Dianhydride |
| 7-ACA | 7-aminocephalosporanic Acid |
| 7-ADCA | 7-aminodesacetoxycephalosporanic Acid |
| A.I. | Active Ingredient |
| A/C | Activated Carbon |
| A/CNK | [Al2O3/(CaO + Na2O + K2O)] (Shand's Index) |
| A/PI | Ab Initio Perturbed Ion (quantum Clacn. Method) |
| A2LA | American Association For Laboratory Accreditation |
| AA | Aluminum Association |
| AA | Assistance Administration |
| AA | Assistant Administrator |
| AA | Activation Analysis |
| AA | Atomic Absorption |
| AAA | Abdominal Aortic Aneurysm |
| AAAI | American Association For Artificial Intelligence |
| AAAS | American Academy Of Arts And Sciences |
| AAAS | American Association For The Advancement Of Science |
| AABW | Antarctic Bottom Water |
| AACA | Ambient Atmosphere Conditions And Aging |
| AACC | American Association For Clinical Chemistry |
| AACC | American Association Of Cereal Chemists |
| AACE | American Association Of Coast Engineers |
| AACG | American Association For Crystal Growth |
| AACI | Association Of American Cancer Institutes |
| AACR | American Association For Cancer Research |
| AACVD | Aerosol-assisted Chemical Vapor Deposition |
| AAD | Atomic Absorption Detector |
| AADA | Abbreviated Antibiotic Drug Application |
| AADO | Angular (momentum Conservation) Average Dipole Orientation Theory (in Io-mol. Reactions) |
| AAEC | Australian Atomic Energy Commission |
| AAEE | American Academy Of Environmental Engineers |
| AAEM | Advanced Analytical Electron Microscope |
| AAETA | Atomic Spectroscopy With Electrothermal Atomization |
| AAFEB | Anaerobic Attached Film Expanded Bed |
| AAFEGE | Asymptotically Adjusted Free-electron-gas Exchange (potential) |
| AAGP | Aerobic Attached Growth Process |