an appliance that allows an object (such as a ship's compass) to remain horizontal even as its support tips
source: WordNet 3.0
A contrivance for permitting a body to incline freely in all directions, or for suspending anything, as a barometer, ship's compass, chronometer, etc., so that it will remain plumb, or level, when its support is tipped, as by the rolling of a ship. It consists of a ring in which the body can turn on an axis through a diameter of the ring, while the ring itself is so pivoted to its support that it can turn about a diameter at right angles to the first.
