c.1600, "a regard or reference" (to something), from Latin retrospectum, past participle of retrospicere "look back," from retro- "back" (see retro-) + specere "look at" (see scope (n.1)). Meaning "survey of past events" is from 1660s.
Looking backward, reflecting on the past. For example, In retrospect, he regarded his move as the best thing he'd ever done. This idiom employs retrospect in the sense of “a view of the past.”
[ Second half of 1600s