Plain English Meaning
How fast the airplane tips its wings from level to a banked position, or from one bank angle to another.
Full Definition
The speed at which an airplane rotates about its longitudinal axis, typically expressed in degrees per second. Roll rate determines how quickly the airplane banks into or out of a turn in response to aileron input.
Context Anchor
Encountered when establishing a turn, rolling out of a turn, or judging how smoothly the airplane is changing bank.
Derivation
Roll' describes rotation around the longitudinal axis (the line running nose to tail), and 'rate' means how fast something happens per unit of time. Together: how fast the airplane rolls.