Plain English Meaning
A turn where you use a watch or timer instead of the heading indicator to know when to stop turning. You hold a steady rate of turn and time it, knowing how many seconds it takes to turn through the heading change you want.
Full Definition
A turn in which the change in heading is controlled by reference to a clock or timing device rather than to the heading indicator, using a known rate of turn (typically standard rate, 3 degrees per second) so that a desired number of degrees of turn is achieved over a calculated number of seconds.
Context Anchor
Used in instrument flying, especially when practicing or using backup methods to change heading without relying only on the heading indicator.
Why Pilots Care
Provides a reliable method to change heading accurately when primary directional instruments are unavailable.