Fathom is a unit of measure that is equal to 6 feet. The name derives from the Old English word fæðm meaning embracing arms or a pair of outstretched arms. In Middle English it was fathme, cognate to the Latin patēre to be open, pandere to spread or unfold, and the Greek petannynai to spread out. Therefore its earlier meaning, which is now obsolete, a full stretch of the arms in a straight line, and consequently grasp or reach.

The length of a fathom has varied over the years. The lengths of 5 feet up to 6.08 feet have been used. The British Admiralty defined a fathom to be a thousandth of an imperial nautical mile, which at the time was 6080 ft. So this is where we find 6.08 feet. In practice the fathom was always regarded as exactly 6 feet. No conflict in the real world existed as depths on Imperial nautical charts were indicated in feet if less than 30 feet and in fathoms for depths above that.

In 1954 the United States and countries of the Commonwealth of Nations defined the length of the international yard to be exactly 0.9144 meters. After this the use of fathoms declined and is basically not used today. Although the curve on the continental shelf is still referred to as the 50 fathom curve.

Fathom is also tied to another Naval term, Deep six. When burials at sea were standard practice for sailors the body was weighted down and required to be buried in depths of water greater than 6 fathoms, or more than 36 feet.