I'll add this to the glossary:
assistance lifts are lifts that assist your main lifts (or key lifts) by emphasising a possible weak spot or simply lending more volume without further taxing the central nervous system on that particular lift. For example, if you are a powerlifter, or use a powerlifting based routine your main lifts are squat, deadlift, and bench press. As such hack squats, front squats, pause squats, etc., Romanian deadlifts, good mornings, SLDLs, zercher deadlifts, etc. and close grip bench, board press, incline press, overhead presses etc. are all common assistance lifts for powerlifters. Also note that there is often carryover for assistance for one exercises to assistance for another (Romanian deadlifts may very well function as assistance for deadlifts or squats). Any exercise you do with the goal of increasing ANOTHER exercise can be considered assisance.