Plain English Meaning
A weather boundary between two different air masses that is sitting still instead of moving across the ground. Because it isn't moving on, the same area can get hours or days of similar weather.
Full Definition
The boundary between two air masses of differing temperature and humidity that is not moving, or is moving very slowly (typically less than about 5 knots). Neither air mass is displacing the other, so the front remains in roughly the same place for hours or days. Weather along a stationary front is often persistent, with extended periods of clouds, rain, or low ceilings.
Context Anchor
Pilots encounter stationary fronts in weather briefings, surface weather charts, and when checking whether weather along a may in place.