| Acronym | Meaning |
| ARAC | Acid Rain Advisory Committee |
| ARAC | Atmospheric Release Advisory Capability |
| ARAES | Angular Resolved Auger Electron Spectroscopy |
| ARAMS | Advanced Regional Atmospheric Modeling System |
| ARAR | Applicable Or Relevant And Appropriate Standards, Limitations, Criteria, And Requirements |
| ARAS | Atomic Resonance Absorption Spectroscopy |
| ARAT | Avion De Recherche Atmospherique Et Teledetection |
| ARB | Air Resources Board (California) |
| ARC | AIDS-related Complex |
| ARC | Accelerating Rate Calorimetry |
| ARC | Advanced Regulatory Control |
| ARC | Antireflection Coating |
| ARCHIE | Automated Resource For Chemical Hazard Incident Evaluation |
| ARCS | Alternative Remedial Contracting Strategy (US EPA) |
| ARCS | Assessment And Remediation Of Contaminated Sediments |
| ARDE | Aspect Ratio Dependent Etching |
| ARDEC | Armament Research |
| ARDRA | Amplified RDNA Restriction Analysis |
| ARDS | Acute Respiratory Distress Syndrome |
| ARDS | Adult Respiratory Distress Syndrome |
| ARDS | Airborne Research Data System |
| ARE | Activated Reactive Evaporation |
| ARE | Androgen-responsive Element |
| AREAL | Atmospheric Research And Exposure Assessment Laboratory |
| AREELS | Angle-resolved Electron Energy Loss Spectroscopy |
| AREP | Averaged Relativistic Effective Potential |
| ARF | ADP-ribosidation Factor |
| ARF | ADP-ribosylation Factor |
| ARF | Absolute Response Factor |
| ARF | Acute Renal Failure |
| ARGOS | Advanced Remote Gaseous Oxides Sensor |
| ARGOS | Advanced Research And Global Observation Satellite |
| ARGOS | Automatically Represents Graphics Of Chemical Structures |
| ARI | Aldose Reductase Inhibitor |
| ARIA | Atm. Response In Aurora |
| ARIAP | Acid Rain Impact Assessment Program |
| ARIC | Acid Rain Information Clearinghouse |
| ARIES | Advanced Reactor Innovation And Evaluation Studies |
| ARIES | Advanced Recovery And Integrated Extraction System |
| ARIES | Angle Resolved Ion Electron Spectroscopy |
| ARIM | Accelerator And Reactor Improvement And Modification |
| ARIMA | Auto Regressive Integrated Moving Average Model |
| ARIMAX | Auto Regressive Integrated Moving Average With Exogeneous Input Model |
| ARIP | Accidental Release Information Program |
| ARIPES | Angle-resolved Inverse Photoemission Electron Spectroscopy |
| ARIPO | African Regional Intellectual Property Organization |
| ARISE | Advanced Radio Interferometry Between Space And Earth |
| ARISS | Angle Resolved Ion Scattering Spectroscopy |
| ARM | Advanced Reservoir Management (petroleum) |
| ARM | Agricultural Runoff Model |