A flat route is an American football route, used in passing plays. A flat route is usually run by a running back or a fullback. When run by a receiver it can be known as a speed out or arrow route. The eligible receiver runs parallel to the line of scrimmage till near the sidelines (in the flat) and turns toward the quarterback to wait for the pass. The QB's pass should arrive when he has not yet passed the line of scrimmage. The receiver will then turn upfield at the sideline and run straight down the field.
The route is used, sometimes as a safety relief if post are covered, with long post, long corner or fly routes, so the safeties and the cornerbacks should be upfield when the pass is caught by the RB or FB. There should be a linebacker covering the RB/FB on these kinds of plays, which is likely to be an easy match for an elusive runner like the running back.