Plain English Meaning
The cruising height a pilot flies at when on an instrument flight plan. The rules say which heights are legal for which directions of travel, so aircraft going opposite ways are kept at different altitudes.
Full Definition
The altitude or flight level assigned to an aircraft operating under Instrument Flight Rules during the en route, level-flight portion of a flight. Under uncontrolled conditions above 3,000 feet AGL, it must be selected according to magnetic course: odd thousands (plus 500 feet for VFR-on-top) for magnetic courses 0° through 179°, and even thousands for magnetic courses 180° through 359°, up to and including 18,000 feet MSL. Above 18,000 feet MSL (), are used following the same odd/even rule based on magnetic course. In , may assign any altitude regardless of direction.