Not sure if everyone does this, but I call the GM controlled opponents 'OpFor' because its not an enemy player in a versus game, its a gamemaster villian of the week thats not really there to defeat the players, just to challenge them as part of a narrative campaign. Players have to track stuff like repairs and experience, while OpFor ceases to exist after the battle, to be replaced with next weeks units.
'The Enemy' I use more when talking about another player's force in versus games where its win/loss, not narrative campaign.