shoot 'em up

Short definition from Wiktionary

Noun

1. 1
   A piece of media which depicts considerable gunplay.
2. 2
   A piece of media which depicts considerable gunplay: A video game in which one plays as a character moving and shooting in an autoscrolling two-dimensional world, with emphasis on dodging bullets and predicting enemy movement patterns.
3. 3
   Alternative spelling of shoot 'em up.