Plain English Meaning

The steady altitude an aircraft holds during the main, level part of a flight — after the climb and before the descent.

Full Definition

An altitude or flight level maintained during en route level flight. It is a constant altitude assigned by ATC or selected by the pilot in compliance with the applicable flight rules.

Context Anchor

You will see this term in flight planning, air traffic control instructions, and discussions of altitude selection during the en route part of a flight.

Derivation

From 'cruise,' originally a sailing term meaning to sail steadily without a fixed destination in mind. In it kept the sense of steady, level travel — the part of the flight between climbing and descending in.