The part of the frame and fork that the wheels are bolted into.

Dropouts in their most common forms are the slotted plates on your frame or fork that your wheels bolt into. They usually come in pairs and they can be found in the front or rear. Some dropouts have vertical slots and some have horizontal slots. They vary widley in shape, size, thickness and general design but they all provide a place to secure the hub axle in some way or another.

The slot in frame or fork that the axle slides in when installing a wheel.

on a bike frame, the slots into which the front and rear wheel axles fit.

Small, slotted openings which hold the front and rear wheel axles. On many bikes the dropouts do not appear to be separate parts. They are merely the openings at the ends of two other frame sections: the seat stays and the front fork.

a slot in the frame into which the rear wheel axle fits

A bicycle dropout (or frame end and fork end) is a slot in a frame or fork where the axle of the wheel is attached.