What is Lord Lyndhurst's Act?

Alternative Forms: Lyndhurst's Loud Act
Legal Definition
This statute (5 & 6 Wm. IV. c. 54) renders marriages within the prohibited degrees absolutely null and void. Theretofore such marriages were voidable merely.
-- Black's Law Dictionary
Legal Definition
An English statute rendering void marriages within certain degrees of relationship.
-- Ballentine's Law Dictionary