Definition: A cubit is an ancient unit of measurement. Like the foot, which still carries a name that reflects its anatomical origin, the cubit (from cubitum Latin for 'elbow') was the measurement from the elbow to the outstretched finger tip. Obviously, this measurement varies considerably from one person to the next.
According to "Measures, Weights, and Values" A Dictionary of the Bible. by W. R. F. Browning. Oxford University Press Inc., there are long and short cubits. A short cubit is 6 hand's breadths = 0.45 meters. A long cubit is 7 hand's breadths = 0.525 meters.