| Acronym | Meaning |
| ARM | Anhysteretic Remanent Magnetization |
| ARM | Area Radiation Monitoring |
| ARM | Atmospheric Radiation Measurement |
| ARM | Atomic Resolution Microscope |
| ARMA | Autoregressive-moving Average Stochastic Model |
| ARMAX | Autoregressive Moving Average Exogenous |
| ARMI | Analytical Reference Materials International |
| ARMS | Angle-resolved Mass Spectrometry |
| ARNT | Aryl Hydrocarbon Receptor Nuclear Translocator |
| ARO | Analytical Radial Orbital |
| ARP | Adiabatic Rapid Passage |
| ARP | Angle-resolved Photoemission |
| ARPA | Advanced Research Projects Agency |
| ARPA | Augmented Random Phase Approximation |
| ARPEFS | Angle-resolved Photoemission Extended Fine Structure |
| ARPES | Angle-resolved Photoelectron Spectroscopy |
| ARPKD | Autosomal Recessive Polycystic Kidney Disease |
| ARPS | Angle-resolved Photoelectron Spectroscopy |
| ARPUS | Angle-resolved Ultraviolet Photoelectron Spectroscopy |
| ARROW | Anti-resonant Reflecting Optical Waveguide |
| ARRP | Acid Rain Research Program |
| ARRP | Ammonia Removal And Recovery Process |
| ARRPA | Air Resources Regional Pollution Assessment Model |
| ARS | Agricultural Research Service (US Dept. Of Agriculture) |
| ARS | Autonomously Replicating Sequence |
| ARSCF | Approximate Relativistic SCF |
| ARSCFC | Approximate Relativistic SCF (with) Corrections |
| ARSPIPES | Angle Resolved Spin Polarised Inverse Photo Emission Spectroscopy |
| ART | Advanced Recycling Technologies |
| ART | Absolute Rate Theory |
| ART | Asphalt Residual Treatment |
| ART | Atom Relay Transistor |
| ARUPS | Angle-resolved Ultraviolet Photoelectron Spectroscopy |
| ARXAS | Angle-resolved X-ray Absorption |
| ARXPD | Angle Resolved X-ray Photoelectron Diffraction |
| ARXPS | Angular Resolved X-ray Photoelectron Spectroscopy |
| AS | Auger Spectroscopy |
| AS | Absorption Spectroscopy |
| AS | Activated Sludge |
| AS | Air Stripping |
| AS-PCR | Allele-specific Polymerase Chain Reaction |
| ASA | American Society Of Agronomy |
| ASA | Acrylonitrile-styrene-acrylate Terpolymer |
| ASA | Alkenylsuccinic Anhydride |
| ASA | Alternancy Symmetry Adapted (method For Spin-independent Hamiltonians For Alternant Hydrocarbons) |
| ASA | At. Sphere Approxn. |
| ASAE | Alkali-sulfite-anthraquinone-ethanol (pulping Process) |
| ASAE | American Society Of Agricultural Engineers |
| ASAS | Advanced Solid State Array Spectrometer |
| ASASP | Active Scattering Aerosol Spectrometer Probe |