A more thorough explanation:

A retrospective law, also known as a retroactive law, is a legislative act that looks back in time and affects acts or facts that existed before the act came into effect. This means that the law applies to events that have already occurred, rather than only to future events.

For example, if a law is passed today that makes it illegal to own a certain type of car, and the law is made retroactive to last year, then anyone who owned that type of car last year would be breaking the law, even though it was legal at the time they owned it.

However, a retroactive law can be unconstitutional if it falls into one of four categories:

1. It is in the nature of an ex post facto law or a bill of attainder
2. It impairs the obligation of contracts
3. It divests vested rights
4. It is constitutionally forbidden

Overall, retrospective laws are controversial because they can be seen as unfair to those who are affected by them. It is important for lawmakers to carefully consider the potential consequences of making a law retroactive before doing so.