1.nounThe straight part of a racetrack, running track, etc., opposite the finishing line; the backstretch.
"He got boxed in amongst four African runners in the back straight of the last lap, and was only able to get out into the clear coming into the home straight."
2.nounAn area next to a racetrack used to stable the racehorses and house employees.
3.nounThe middle part of an event.
4.noun(exercise) An exercise that stretches the muscles of the back.