Plain English Meaning
A small indicator on the cockpit screen that shows how fast you are turning, not just which way you are banked.
Full Definition
A display element on an electronic flight display that shows how quickly the airplane is turning, measured in degrees per second. It typically appears as a small pointer or trend marker near the heading or bank indication, and is calibrated so that a full deflection corresponds to a standard rate turn of 3 degrees per second.
Context Anchor
Seen on an electronic flight display while using bank control during instrument flying.
Derivation
“Turn” means a change in direction. “Rate” means an amount measured over time. “Indicator” comes from a word meaning “to .” Together, the term means a display that points how much the is turning over time.