'Cubit' definitions:

Definition of 'cubit'

noun

An ancient unit of length based on the length of the forearm

From: GCIDE

Cubit \Cu"bit\ (k[=u]"b[i^]t), n. [L. cubitum, cubitus; elbow, ell, cubit, fr. (because the elbow serves for leaning upon) cubare to lie down, recline; cf. Gr. ky`biton elbow, ky`ptein to bend, stoop, kyfo`s bent, stooping, humpbacked. Cf. Incumbent, Covey.]
1. (Anat.) The forearm; the ulna, a bone of the arm extending from elbow to wrist. [Obs.] [1913 Webster]
2. A measure of length, being the distance from the elbow to the extremity of the middle finger. [1913 Webster]
Note: The cubit varies in length in different countries, the Roman cubit being 17.47 inches, the Greek 18.20, the Hebrew somewhat longer, and the English 18 inches. [1913 Webster]

Cubit Heb. 'ammah; i.e., "mother of the arm," the fore-arm, is a word derived from the Latin cubitus, the lower arm. It is difficult to determine the exact length of this measure, from the uncertainty whether it included the entire length from the elbow to the tip of the longest finger, or only from the elbow to the root of the hand at the wrist. The probability is that the longer was the original cubit. The common computation as to the length of the cubit makes it 20.24 inches for the ordinary cubit, and 21.888 inches for the sacred one. This is the same as the Egyptian measurements.
A rod or staff the measure of a cubit is called in Judg. 3:16 _gomed_, which literally means a "cut," something "cut off." The LXX. and Vulgate render it "span."