Fathom

A fathom is a unit of length in the imperial and the U.S. customary systems equal to 6 feet, used especially for measuring the depth of water. The fathom is not an International Standard unit, nor is it accepted internationally as a non-SI unit. However it is historically the most frequently employed maritime measure of depth in the English speaking world. There are two yards in an imperial fathom. Originally the span of a man's outstretched arms, the size of a fathom has varied slightly depending on whether it was defined as a thousandth of an nautical mile or as a multiple of the imperial yard. Formerly, the term was used for any of several units of length varying around 5–5 ¹⁄₂ feet.