backfield

Short definition from Wiktionary

Noun

1. 1
   The rear part of the field of play, particularly: Synonym of outfield.
2. 2
   The rear part of the field of play, particularly: The area behind the line of scrimmage occupied before the snap by the team on offense.
3. 3
   The rear part of the field of play, particularly: The area behind either the offensive or defensive line.
4. 4
   The players occupying this area.