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

[1] 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.

[7] By the Byzantine period, this unit came in two forms: a "simple orguia" (ἁπλὴ ὀργυιά, haplē orguiá) roughly equivalent to the old Greek fathom (6 Byzantine feet, c. 1.87 m) and an "imperial" (βασιλικὴ, basilikē) or "geometric orguia" (γεωμετρικὴ ὀργυιά, geōmetrikē orguiá) that was one-eighth longer (6 feet and a span, c. 2.10 m).

[11] Other definitions of fathom include: One metre is about 0.5468 fathoms 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.

[17] To measure the depth of shallow waters, boatmen used a sounding line containing fathom points, some marked and others in between, called deeps, unmarked but estimated by the user.

Especially in Pacific coast fisheries the setline was composed of units called skates, each consisting of several hundred fathoms of groundline, with gangions and hooks attached.

[25] A kite was a drag, towed under water at any depth up to about 40 fathoms (240 ft; 73 m), which upon striking bottom, was upset and rose to the surface.

A shot, one of the forged lengths of chain joined by shackles to form an anchor cable, was usually 15 fathoms (90 ft; 27 m).

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

[2] In Britain, it can mean the quantity of wood in a pile of any length measuring 6 feet (1.8 m) square in cross section.

In Hungary the square fathom ("négyszögöl") is still in use as an unofficial measure of land area, primarily for small lots suitable for construction.