Sheet
F5. The Reference Layer: Glossary, Units, Signals and Information SourcesDefinition
Line controlling the angle of a sail.
A sheet is the running-rigging line that controls a sail’s set by adjusting the angle and tension of its clew or foot to the wind. The mainsheet trims the boom; the jib and genoa sheets (port and starboard) haul the headsail clew aft after each tack; the spinnaker has a sheet and a guy. Hauling in (trimming) flattens and brings the sail inboard for upwind sailing; easing lets it out for reaching and running. The word names the line, not the sail; to be three sheets to the wind is to have them all flying loose.
Source: Standard sailing terminology; Royce's Sailing Illustrated and Skene's Elements of Yacht Design.