Unforgivable Curses are curses seen in the Harry Potter Universe.
The unforgivable curses are the most powerful curses in the wizarding world. They are tools of the Dark Arts, and where first classified Unforgivable in 1717. Unforgivable curses are powerful meaning there is great power, and great skill needed to cast them. The three curses seemed to be called "unforgivable" because they carry an automatic life sentence in Azkaban. Aurors where permitted to use them during the First Wizard war, possibly as a means of last resort. Lord Voldemort legalised the curses when he took control of the Ministy.
All three curses are seen in Harry Potter and the Goblet of Fire. The killing curse is seen more than the others, being cast or attempted to cast in all films.