| Domain | Definition | ||
| Aerospace | Horizontal Reference Line. (references) | ||
| Military | Hughes Research Laboratories. (references) | ||
| Transportation | HRL Laboratories, LLC (used to be Hughes Research Laboratories). (references) | ||
| Weather | The Hydrological Research Laboratory at the Office of Hydrology (OH). (references) | ||
Source: compiled by the editor from various references; see credits. | Top | ||
