The Collaborative International Dictionary of English v.0.48:
off-peak \off-peak\ adj. Having less than maximal use or demand or activity; -- of a period of time occurring as a defined part of a time cycle; as, off-peak telepone rates are available at night and on weekends; off-peak fares. [Narrower terms: off-season ] {peak [WordNet 1.5 +PJC]