Plain English Meaning
It's the little arrow mark on the turn instrument. Line the moving part up with that mark and you know you're turning at the standard rate.
Full Definition
On a turn coordinator or turn-and-slip indicator, the small arrow-shaped pointer or symbol used as the reference mark that the miniature aircraft (or needle) aligns with to show a standard-rate turn. When the wing of the symbolic aircraft, or the needle, points to the arrowhead, the aircraft is turning at three degrees per second.
Context Anchor
Seen on instrument displays that show turn rate, especially during instrument flying when the pilot is checking the rate of a turn.