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.

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 5–51⁄2 feet (1.5–1.7 m).

The name derives from the Old English word fæðm, corresponding to the Old High German word "fadum" meaning embracing arms or a pair of outstretched arms. In Middle English it was fathme.

One fathom is equal to:

1.8288 metres exactly (1 metre is about 0.5468 fathoms)

2 yards (1 yard is exactly 1/2 of a fathom)

6 feet (1 foot is exactly 1/6 of a fathom)

18 hands

72 inches

In the international yard and pound agreement of 1959 the United States, Australia, Canada, New Zealand, South Africa, and the United Kingdom defined the length of the international yard to be exactly 0.9144 metre.

More Info: en.wikipedia.org