Plain English Meaning
A turn with a moderate amount of tilt, where the airplane stays at that tilt by itself without you having to hold the control wheel into or out of the turn.
Full Definition
A turn flown at a bank angle of approximately 20 to 45 degrees, in which the airplane maintains the bank without the aileron input required to hold it. Below this range, the airplane tends to roll back toward wings-level on its own; above it, the airplane tends to overbank and steepen the turn if the controls are released.
Context Anchor
Seen in airplane maneuver descriptions, including stall demonstrations and turns in the traffic pattern.