A part of the day during which traffic congestion on roads and crowding on public transport is at its highest. Normally, this happens twice a day—once in the morning and once in the evening, the times during when the most people commute. The term is very broad but often refers to specifically private automobile transportation traffic, even when there is a large volume of cars on a road but not a large number of people, or if the volume is normal but there is some disruption of speed.

The name is sometimes a misnomer, as the peak period often lasts more than one hour and the "rush" refers to volume of traffic, not rate of flow. Typically, rush hour is 6–10 am (06:00–10:00) and 4–7 pm (16:00–19:00). Some places may experience another, less frantic, lunchtime rush hour from noon to 2 pm (14:00).

The frequency of public transport is usually higher in the rush hour, and longer trains, or larger vehicles are often employed. However, the increase in capacity is often less than the number of passengers, due to the limits on available vehicles, staff and, in the case of rail transport, track capacity including platform length. The resulting crowding may force many passengers to stand, and others may be unable to board. This can make public transport less attractive, leading to higher car use, partly shifting the congestion to roads.