Gaff-topsail n. : A small triangular sail having its foot extended upon the gaff and its luff upon the topmast.

Example:


Taxonomy
topsail, sail, Gaff topsails, vessels, square, gaff topsail, masts, square sails, gaff, topsail schooners, topsail schooner, free encyclopedia, sailing, topmast, jack, square-rigged, Gaff rig, rigging.

Defination from the web:
gaff topsail is generally a triangular sail set between the gaff and the top of the mast or topmast. ... A jack-yard topsail (or Club topsail) instead has its lower edge (or foot) extended out beyond the end of the gaff with a short yard, called a "jack-yard" ...