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word-forming element meaning "backwards; behind," from Latin retro (prep.) "backward, back, behind," also of time, "formerly, in past times," probably originally the ablative form of *reteros, based on re- "back" (see re-).
L. retro stands to re- as intro, "in, within"; to in, "in," and as citro, "hither," stands to cis, "on this side." [Klein]Common in combinations in post-classical Latin (the classical equivalent was post-). Active in English as a word-forming element from mid-20c.
Backward; back: retroposition.
Situated behind: retroperitoneum.