A bollard is a short post, especially one used for mooring a boat. If you visit a working harbor, you'll see many bollards.
Bollards are known for being especially sturdy, strong enough that a boat securely moored there can't be blown or tossed around. Some ships also have bollards on board, for wrapping and storing unused rope. Posts used to control automobile traffic are sometimes also called bollards. The root is thought to be the Old Norse bolr, "tree trunk."