Plain English Meaning
It is an invisible ramp that climbs gently away from the end of the runway. Procedure designers check whether anything sticks up through that ramp. If something does, the departure has to be adjusted so airplanes climb steeply enough to stay above it.
Full Definition
An imaginary sloped surface used in instrument departure procedure design that rises one foot vertically for every 40 feet horizontally from the end of the runway. Obstacles that penetrate this surface must be either removed, marked, lighted, charted, or accounted for by requiring a higher-than-standard climb gradient on the departure procedure.
Context Anchor
Seen in instrument departure and procedure-design discussions, especially when explaining how obstacle departure procedures are created.