Plain English Meaning
A button on the cyclic stick that lets the pilot reset the stick to its center position, so they have a clean starting point for the next control input.
Full Definition
A trim device on a helicopter cyclic control that, when pressed, releases or neutralizes the cyclic force gradient so the cyclic stick returns to a centered (neutral) position, allowing the pilot to re-establish a known reference point for cyclic input.
Context Anchor
Seen in helicopter instrument flying when adjusting cyclic trim after setting the desired attitude.