Mainsail
From MITNA
The mainsail is the designation for a specific sail on a sailboat. If a boat only has one sail, that is the mainsail. The luff of the mainsail is attached to the aft side of the tallest mast of the boat, and the foot of the mainsail is usually attached to a boom.
Note that on some boats, the genoa can be larger than the mainsail, so the mainsail is not always the largest sail on a boat.
