A yaw is a swerve off course. On a sailboat, an unexpected yaw can make you lurch to one side — and it might also make you seasick.
Both as a noun and a verb, yaw basically means "swerve or twist." It's most common for this word to be used in discussions of a vehicle's movement, especially ships, airplanes, and spacecraft. A pilot might describe a jet that tends to yaw to the right, while a boat captain might warn her crew that an approaching storm will cause the ship to pitch and yaw. Etymologists believe that yaw and yacht share a root.