The Bushwhacker is a Combat Role of someone who uses Stealth and Hit and Run tactics to engage the enemy quickly, unload from point blank while their defenses are down, and then disappear before they can regroup or counterattack.
In general, this will overlap with at least one other combat role, depending on whether you rely mostly on cleverness, camoflauge, or quickness to get the job done. See also The DPSer, The Ninja, Glass Cannon, The Nuker, Evasion Tank. For the effectiveness of such tactics, see Sneak Attack, Fighting Dirty, and The Drop.
Bushwhacking is a common tactic for Ace Pilots, because hit-and-run is frighteningly effective when you're zipping around at supersonic speeds, and the enemy is precariously defying gravity in a big hunk of metal.