| Abbreviation | Definition |
|---|---|
| IXO | International X-ray Observatory |
| IXS | Interstellar Experimental Ship |
| J | Joule (SI Unit) |
| J/M | Jettison Motor |
| JA | Jupiter Approach |
| JACD | Joint Architectural Control Document |
| JACEE | Japanese-American Cooperative Emulsion Experiment |
| JAD | Joint Assembly Demonstration |
| JAEIC | Joint Atomic Energy Intelligence Committee |
| JAN | Joint Army-Navy |
| JANNAF | Joint Army, Navy, NASA, and Air Force |
| JAS | Japan Amateur Satellite |
| JAS | Journal of Aerospace Science |
| JAS | Journal of Atmospheric Science |
| JATO | Jet Assisted Take Off |
| JAXA | Japanese Aerospace Exploration Agency |
| JBOSC | Joint Base Operations and Support Contract |
| J-BOSC | Joint Base Operations and Support Contract |
| JBP | Jettison Booster Package |
| JCL | Job Control Language |