Plain English Meaning
An instrument on the panel that tells the pilot which way the airplane is turning and how fast it is turning.
Full Definition
A flight instrument that shows the rate and direction of the airplane's turn about its vertical axis (yaw), and, in combination with an inclinometer ball, whether the turn is coordinated. It typically appears as either a turn-and-slip indicator (with a needle) or a turn coordinator (with a small symbolic airplane), and is driven by a gyroscope.
Context Anchor
Seen during taxi when the pilot checks that flight instruments respond correctly before takeoff.