אַמָּה ʼammâh (H520)

properly, a mother (i.e. unit of measure, or the fore-arm (below the elbow), i.e. a cubit; also a door-base (as a bond of the entrance)

1) cubit-a measure of distance (the forearm), roughly 18 in (. 5m). There are several cubits used in the OT, the cubit of a man or common cubit (De 3:11), the legal cubit or cubit of the sanctuary (Eze 40:5) plus others. See a Bible Dictionary for a complete treatment.

prolonged from אֵם (H517);