From Longman Business Dictionary
off-peakˌoff-ˈpeak adjectiveadj [only before a noun]1TRAVELoff-peak travel, electricity etc is cheaper than normal because fewer people are travelling, using electricity etc at that timediscounts on off-peak travel
2COMMERCEoff-peak hours or periods are the times when fewer people want to do or use something, and when it is usually cheaperTelephone charges are lower during off-peak periods.
→ compare peak2 —off-peak adverbadvIt is much cheaper to travel off-peak.