- A Ampere
- A/UX Apple UniX
- AA Auto Answer / Add Accelerator
- AACE Association for the Advancement of Computing in Education
- AARP AppleTalk Address Resolution Protocol
- ABIOS Advanced BIOS
- ABP Alternate BiPolar
- AC Alternating Current
- ACDI Asynchronous Communication Device Interface
- ACID Atomic, Consistent, Isolated, Durable
- ACK ACKnowledgement
- ACL Access Control List
- ACM Association of Computing Machinery / Audio Compression Manager
- ACS Asynchronous Communication Server
- ACTLU ACTivate Logical Unit
- ACTPU ACTivate Physical Unit
- ACTT Advanced Communications and Timekeeping Technology
- ACU Access Control Units
- AD Administrative Domain
- ADF Automatically Deferred Function
- ADPCM Adaptive Differential Pulse-Code Modulation
- ADSP AppleTalk Data Stream Protocol
- ADSS Apple Data Stream Protocol
- AEP AppleTalk Echo Protocol
- AFI Authority and Format Indicator
- AFM Added Font Metrics
- AFP Apple Filing Protocol
- AFRP Arcnet Fragmentation Protocol
- AFS Andrew File System
- AGS Asynchronous Gateway Server
- AGU Address Generation Units
- AI Artificial Intelligence
- AIR Apple Internet Router
- AIX Advanced Interactive EXecutive
- ALGOL ALGOrithmic Language
- ALMS Appware Loadable ModuleS
- ALU Arithmetic Logic Unit
- AM Active Monitor
- AMD Advanced Micro Devices
- AMI Alternate Mark Inversion
- AMP Apple Management Protocol
- AMT Address Mapping Table
- ANS Advanced Network System
- ANSI American National Standards Institute
- AO/S Agents Object System
- AOCE Apple Open Collaboration Environment
- AOL America OnLine
- APA All Points Addressable
- API Application Programming Interface
- APL A Programming Language
- APM Advanced Power Management
- APPC Advanced Program-to-Program Communications
- APPN Advanced Peer-to-Peer Networking
- ARA Apple Remote Access
- ARAP AppleTalk Remote Access Protocol
- ARCNET Attached Resource Computer NETwork
- ARE All Routes Explorer
- ARI Address Recognized Indicator Bit
- ARIES ATM Research & Industrial Enterprise Study
- ARP Address Resolution Protocol
- ARPA Advanced Research Projects Agency
- ARPANET Advanced Research Projects Agency NETwork
- AS Autonomous System
- ASAD Association of Software Authors and Distributors
- ASC Advanced Software Concepts
- ASCII American Standard Code for Information Interchange
- ASI Adapter Support Interface
- ASIC Application-Specific Integrated Circuit
- ASN.1 Abstract Syntax Notation 1
- ASP Association of Shareware Professionals / Association of Shareware Programmers / AppleTalk Session Protocol
- ASPI Advanced SCSI Programming Interface
- AST Automatic Scheduled Testing
- ASTRAL Alliance for Strategic Token Ring Advancement and Leadership
- AT Advanced Technology
- AT&T American Telephone & Telegraph
- ATDT ATtention Dial Tone
- ATM Asynchronous Transfer Mode / Adobe Type Manager
- ATOB ASCII TO Binary
- ATP AppleTalk Transaction Protocol
- ATT Automatic Toll Ticketing
- AUI Attachment Unit Interface
- AUP Acceptable Use Policy
- AURP Apple Update Routing Protocol
- AVI Audio Video Interleave
- AWG American Wire Gauge
- AWK Ano, Weinberger and Kernighan