Yacht
A sailboat or powerboat used for pleasure, not a working boat.
Yard
A spar attached to the mast and used to hoist square sails.
Yard arm
The end of a yard.
Yaw
Swinging off course, usually in heavy seas. The bow moves toward one side of the intended course.
Yawl
A two-masted sailboat with the shorter mizzen mast placed aft of the rudder post. A ketch is similar, but the mizzen mast is forward of the rudder post.