Plain English Meaning
The altitudes printed on instrument flight charts that tell a pilot how high, how low, or exactly what altitude to fly along a route or procedure.
Full Definition
The published altitudes shown on IFR enroute and approach charts that define the vertical limits within which an aircraft may legally and safely fly along a given route or procedure. These include minimum, maximum, and mandatory altitudes that account for terrain clearance, obstacle avoidance, communication and navigation signal reception, and air traffic separation requirements.