Retrospective law: may impair vested rights but can cure inadvertent statutory defects to prevent windfalls. Definitions distinguish retrospective/retroactive laws that affect contracts, reopen closed transactions, impair vested rights, or alter procedure; a key legal distinction is between true retroactivity (applying new rules to completed transactions) and quasi-retroactivity (applying to transactions in process), with courts focusing on whether the transaction was completed. Legislatures may retrospectively cure inadvertent statutory or administrative defects to prevent windfalls, but such curative measures must be weighed against protection of vested rights and the specific facts of past transactions.