(RETRO: backward, back, behind; SPEC: look at; examine) is thinking about or looking back at the past
relating to or thinking about the past:
a retrospective album of solo Freddie Mercury tracks
Example: Let’s take a retrospective look at the popular 90s sitcom, Friends.
(ACTIVE: to drive, draw out or forth, move) is when something is done NOW but made effective from a time in the past.
If a law or decision, etc. is retroactive, it has effect from a date before it was approved:
the first British law to have retroactive effect
Example: The tax-free retroactive minimum wage increase is effective immediately.
The nine percent increase will see the basic wage bar move from $64,000 to $70,000 weekly. Citizens will retroactively receive payment at the new rate from January 2019 to the present.
tldr: Retroactive affects the past. Retrospective only looks at the past.
NOTE: In a legal context, a retrospective statute affects the future only. It considers a past event, but affects only the future.