| X | roll axis |
| X- | X-axis |
| X-REF | X-band RF electronics |
| X-SAR | X-band synthetic-aperture radar |
| XEV | Xerox encryption unit |
| XL | X-axis of Spacelab |
| XO | X-axis of orbiter |
| XP | X-axis of payload |
| XPS | X-ray photoelectron spectroscopy |
| XS | X-axis of solid rocket booster |
| XT | X-axis of external tank |