What is a topping lift?

The topping lift (more rarely known as an uphaul) is a line which applies upward force on a boom on a sailboat. Part of the running rigging, topping lifts are primarily used to hold a boom up when the sail is lowered. This line would run from near the free end of the boom forward to the top of the mast. The line may be run over a block at the top of the mast and down to the deck to allow it to be adjusted.