cubit

n. ancient measure of length (equal to 17 to 21 inches or 43 to 53 cm)

For the unit of information, see qubit. For the bone, see ulna. Cubit is the name for any one of many units of measure used by various ancient peoples and is among the first recorded units of length.The cubit is based on measuring by comparing – especially cords and textiles, but also for timbers and stones – to one's forearm length. The Egyptian hieroglyph for the unit shows this symbol. It was employed consistently through Antiquity, the Middle-Ages up to the Early Modern Times.The distance between thumb and another finger to the elbow on an average person and measures about 24 digits or 6 palms or 1½ foot. This is about 45 cm or 18 inches. This so-called "natural cubit" of 1½ foot is used in the Roman system of measures and in different Greek systems.
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Noun
1. an ancient unit of length based on the length of the forearm
(hypernym) linear unit

(n.)
The forearm; the ulna, a bone of the arm extending from elbow to wrist.
  

(n.)
A measure of length, being the distance from the elbow to the extremity of the middle finger.
  

See: Weights And Measures AND Measures