A particular period of the day, typically when people are travelling to or from work, when traffic on the road is particularly heavy or congested. Hyphenated if used as a modifier. I start and finish work late so I can avoid rush hour. It would normally take me about 15 minutes to drive there, but during rush hour, it can take 45 minutes or more. You'd better leave soon if you don't want to get caught in rush-hour traffic.