In nautical contexts, a cleat is a device for securing a rope.
Types of cleat designs include the following:
A cleat hitch is a knot used to secure a rope to a cleat.
A line tied with a cleat hitch to a horn cleat[1] on a dock. The line comes from a boat off the top of the picture, around the right horn, around the left horn, across the cleat from top left to bottom right, around the right horn, and then hitches around the left horn. Note that this line is tied improperly; the line from the boat should initially run to the far (left) side of the horn cleat rather than close (right) side.
A jam cleat. The rope is free to run around the lower part of the cleat (below the screws). The upper part is tapered so that the space between the cleat and the boat gets smaller closer to the screws, causing the rope to jam in the cleat when it is pulled downward.
A large mooring cleat
Cam cleat: the rope passes between two cams, which resist a pull in a direction away from the camera.