The Collaborative International Dictionary of English v.0.48:
Retro- \Retro-\ [L. retro, adv., backward, back. Cf. Re-.] A prefix or combining form signifying backward, back; as, retroact, to act backward; retrospect, a looking back. [1913 Webster]WordNet (r) 3.0 (2006):
retro adj 1: affecting things past; "retroactive tax increase"; "an ex-post-facto law"; "retro pay" [syn: ex post facto, retroactive, retro] n 1: a fashion reminiscent of the past