A fathom is a unit of length in the Imperial system (and the derived U.S. customary units). The name derives from the Old English word fæðm meaning 'a pair of outstretched arms'. In Middle English it was fathme. Its size varied slightly depending on its base definition, either based on a thousandth of an (Admiralty) nautical mile or as a multiple of the imperial yard. There are 2 yards (6 feet) in a fathom.