Trooper was a term for foot-soldiers in armies across time and space, such as the Sontarans, the Judoon, and humans.
In response to the heavy casualties suffered during the Dalek-Movellan War, the Daleks, led by the Supreme Dalek at that time, hired human or humanoid mercenaries to fight for them as Dalek Troopers. (TV: Resurrection of the Daleks)
In the 52nd century, Dorium Maldovar procured security software for the headless monks by extracting it from the memory of a Judoon trooper. (WC: Brain Trafficking)
"Trooper Turlough" became so under the mental control of Eldrad. (AUDIO: Eldrad Must Die!)