| Acronym | Meaning |
| API | A Pleasing Interface |
| API | Absolute Position Indication |
| API | Accountable Property Inventory |
| API | Acquisition Program Integration |
| API | Advanced Passenger Information |
| API | Aerial Photographic Interpretation (American Society of Photogrammetry and Remote Sensing) |
| API | After Project Initiation |
| API | Age Participation Index (UK) |
| API | Air Photo Interpretation |
| API | Aircraft Platform Interface |
| API | Aircrew Position Indicator (USAF) |
| API | Allocation de Parent Isolé (French: Single Parent Allowance) |
| API | Annual Plan for Instruction |
| API | Application Interface (Department of State) |
| API | Application Programmers Initiative |
| API | Applications, Programs and Indentures |
| API | Arab Peace Initiative |
| API | Armor-Piercing Incendiary (ammunition) |
| API | Aromatic Polyimide Insulation |
| API | Asian and Pacific Islander (US Census usage) |
| API | Asset Priority Index |
| API | Asylum Policy Instruction (UK) |
| API | Automation Proficiency Improvement (US Postal Service) |
| API | Average Price Index (describes growth rate, related to inflation measurements) |
| API | Aviation Preflight Indoctrination |