Non-retroactivity is the legal principle that laws do not apply retroactively, whether international laws such as treaties or in criminal law.

What does retroactive to meaning?

: extending in scope or effect to a prior time or to conditions that existed or originated in the past especially : made effective as of a date prior to enactment, promulgation, or imposition retroactive tax.

What does it mean to apply retroactively?

adjective. If a decision or action is retroactive, it is intended to take effect from a date in the past.