Someone who has gone from pretard to full retard and come full circle to being 'normal' again. A person who used to be incapable of normal life functions, but now is considered stable. Few people actually make the progression to post-tard. Post-tards are few and far between, but they exist.
Bill Clinton, Whitney Houston, Charlie Sheen
Whitney Houston is the classic example of a post-tard because of her drug-related past and her re-entering into normal society.