A drop seat, also known by many other names such as "access hatch", "trap door", "butt flap", "seat flap" or "fireman's flap", is a button-up flap in the seat of one-piece garments such as drop-seat pajamas and union suits.
The flap can be unbuttoned to expose the wearer's buttocks. Such a flap is made for toilet use (because the garment is one-piece and so has no separate pants which could be lowered), but can also be used for other things such as bare bottom spanking.
The flap is traditionally closed with buttons, though in modern versions it can also be closed with Velcro fasteners. In loosely cut garments such as pajamas, the seat sometimes has just a horizontal opening which is elastic enough to be pulled down for toilet use - so it's not really a flap.
There are also other garments that have a slit opening in the rear, such as: