1 To lay a sailing ship on the wind with its helm a-lee and its sails shortened and so trimmed that as it comes up to the wind it will fall off again on the same tack and thus make no headway. Vessels normally heave to when the weather is too rough and the wind too strong to make normal sailing practicable. A powered vessel can similarly heave to by heading up into the sea and using its engines just enough to hold it in position. The whole idea in heaving to is to bring the wind on to the weather bow and hold the ship in that position, where it rides most safely and easily. Heaving to is also known as lying a-try or lying to.
2 When a ship is ordered to heave to it is being told to stop, usually so that it can be boarded.