A small open-top shelter, originally a cask, on the top of the foremast, large enough to accommodate a lookout. It was used by whalers to watch for a blow (spout), or in icebound waters to seek a channel
platform that is partially enclosed placed high on or near a ship's mast and is used as a lookout
On a ship, the crow's nest is a small platform high up on the mast, where a person can go to look in all directions. a small box at the top of a ship's mast from which someone can watch for danger, land etc