A secondary exercise chosen to support the main lifts, either by building a muscle they depend on or by fixing a specific weak point.
The main lift is the goal; accessories are the maintenance. A bench presser whose triceps fail first does close-grip bench and pushdowns. A deadlifter who rounds at the start does deficit pulls and rows.
Choose them from evidence, not habit. Watch where the lift breaks down or read the log for which lift stalled, then pick the accessory that trains that. If nothing is broken, accessories default to balance work: hamstrings, upper back, rear delts, cuff.
Keep them at three to four exercises per session, 2 to 4 sets each, at RPE 7 to 9. Accessories that leave a client too tired for next session main lift are no longer accessories.