What is Alea?

Legal Definition
; civil law. The chance of gain or loss in a contract. This chance results either from the uncertainty of the thing sold, as the effects of a succession; or from the uncertainty of the price, as when a thing is sold for an annuity, which is to be greater or less on the happening of a future event; or it sometimes arises in consequence of the uncertainty of both. 2 Duv. Dr. Civ. Fr. n. 74.
-- Bouviers Law Dictionary
Legal Definition
Lat. In the civil law. A game of chance or hazard. Dig. 11, 5, 1. See Cod. 3, 43. The chance of gain or loss in a contract
-- Black's Law Dictionary