What is Fathom?

Legal Definition
A fathom is a unit of length in the imperial and the U.S. customary systems equal to 6 feet (1.8288 metres), used especially for measuring the depth of water.

There are two yards (6 feet) in an imperial fathom. Originally based on the distance between 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 12 feet (1.5–1.7 m).

-- Wikipedia
Legal Definition
A measure of length, equal to six feet. The word is probably derived from the Teutonic word fad, which signifies the thread or yarn drawn out in spinning to the length of the arm, before it is run upon the spindle. Webster; Minsheu. See Ell. Vide Measure.
-- Bouviers Law Dictionary
Legal Definition
A nautical measure of sir -feet in length. Occasionally used as a superficial measure of land and in mining, and in that case it means a square fathom or thirty-six square feet Nahaolelua v. Kaaa-hu, 9 Hawaii, 601.
-- Black's Law Dictionary
Legal Definition
Six feet.
-- Ballentine's Law Dictionary