Plain English Meaning
A safe altitude printed inside each grid square on a chart. If you fly at or above that number anywhere in the square, you are guaranteed to clear all the terrain and obstacles inside it.
Full Definition
An altitude shown on Jeppesen en route charts within a latitude/longitude grid block (typically one degree by one degree) that provides obstacle clearance over the entire block. The Grid MORA gives 1,000 feet of clearance above the highest terrain or obstacle in that block where the highest elevation is 5,000 feet MSL or lower, and 2,000 feet of clearance where the highest elevation is above 5,000 feet MSL. Grid MORAs are not flight-checked for navigation signal reception and are , not -assigned .