Lex commissoria
Black's Law Dictionary · Henry Campbell Black, M.A. · 1910
Black's Law Dictionary
A law by which a debtor and creditor might agree (where a thing bad been pledged to the latter to secure the debt) that, if the debtor did not pay at the day, the pledge should become the absolute property of the creditor.
2 Kent. Comm. 583.
This was abolished by a law of Constantine. A law according to which a seller might stipulate that, if the price of the thing sold were not paid within a certain time, the sale should be void.
Dig. 18, 3.