Plain English Meaning
The airplane is flying straight, with neither wing tipped down. Looking out the front, the horizon runs flat across the windscreen.
Full Definition
An aircraft attitude in which both wings are held parallel to the horizon, with no bank angle in either direction. In this attitude the lift produced by each wing is equal and the aircraft is not turning.
Context Anchor
Used when describing recovery from a slip, normal straight flight, or any moment when the pilot needs the airplane not tilted or .