What does it mean to say that a particular statute is "retroactive" in application?

State courts often have been more inclined than federal courts to invalidate a statute because it applied retroactively.What is the standard or approach principally used by the federal courts when the constitutionality of a retroactive federal statute is challenged under the Due Process Component of the Fifth Amendment?

In your estimation how should courts resolve retroactive challenges to statutes?For example, should retroactivity always be prohibited?Never prohibited?Permitted but only for a fixed period of time (if so, what period)? Permitted only in certain situations (if so, what situations)?Give reasons for your preference.