A control line used to hold up various spars: the main boom and the spinnaker pole are two common examples.
Rigging Topsail Schooner Ships , Rigging
a line that holds up the boom when it is not being used, also the line that controls the height of a spinnaker pole
string from mast down to rear end of boom, to hold boom up. Models (unlike full size boats) have one on the jib boom to control twist in jib
A wire rope or tackle extending from the head of a boom to a mast, or to an elevated part of the ship's structure, for the purpose of supporting the weight of the boom and its loads, and permitting the boom to be raised or lowered.
The topping lift (more rarely known as an uphaul) is a line which is part of the rigging on a sailboat; it applies upward force on a spar or boom. The most common topping lift on a modern sailboat is attached to the spinnaker pole.