0 (on a boat or ship) the most important sail (= a sheet of material attached to a pole, to catch the wind and make the boat or ship move), which is placed on the mainmast:
To make matters worse, the sailboat, their only hope of rescue, had a torn mainsail and was steering with only a jib.
Cord by cord, Harris loosens the mainsail until the great white sheet bellies out with the breeze.
As they watched, they saw the tall red mainsail crumple and fall in great folds on the top of the cabin.
Without the shield of the main sail the sun is suddenly uncomfortably hot.
