Foul is a nautical term meaning to entangle or entwine, and more generally that something is wrong or difficult. The term dates back to usage with wind-driven sailing ships.

It is usually applied to the state of an anchor, which has become hooked on some impediment on the seafloor, or has its cable wound round the stock or flukes. The term is generally utilized when speaking of items of historical value such as the US Navy chief petty officer emblem. The foul anchor is also the official seal of the Lord High Admiral of the United Kingdom. The seal is flown on the ship carrying the monarch to sea. It is also flown during the launching of a warship of the Royal Navy. The foul anchor is also depicted on the emblem of the U.S. Public Health Service. Ships would display a foul anchor to indicate to officers on shore that sick persons were on board, prompting further investigation and possible quarantine.^{[citation needed][clarification needed]}

The term can be applied to many nautical situations: