c.1600, from L. retrospectum, pp. of retrospicere "look back," from retro- "back" + specere "look at" (see scope (1)). Retrospective (adj.) is attested from 1660s; as a noun, it is recorded from 1964, short for retrospective exhibition (1919), etc.
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]