Chiefly used in measuring the depth of water, a fathom is a unit of length equal to which distance?

A fathom is a unit of length equal to six feet (1.828 m) used especially for measuring the depth of water - sometimes used in the singular when qualified for a number (i.e. five fathom deep). It is a unit in the imperial and the U.S. customary systems and is not an International Standard (SI) unit, nor is it accepted as a non-SI unit. It is historically the most frequently used maritime measure of depth in the English speaking world.

There are two yards (6 feet/1.828 m) 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 (Admiralty) 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 (1.5-1.7 m).

For example, a burial at sea (where the body is weighted to force it to the bottom) requires a minimum of six fathoms of water (36 ft/10.972 m). This is the origin of the phrase, "to deep six" as meaning to discard, or dispose of.

The phrase is echoed in William Shakespeare's "The Tempest", where Ariel tells Ferdinand, "Full fathom five thy father lies".