The dropouts are the slots in the forks and the rear triangle where the wheel axles attach.

Small, slotted openings which hold the front and rear wheel axels. 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.

the U-shaped slots that accept the wheel axle.

This does not refer to tattooed downhillers who found sixth grade too tough. Dropouts are the little metal tabs on your frame and fork that keep your wheels where they should be. There are several kinds of dropouts available at the moment. Standard dropouts are found on most cross country bikes and forks. QR20 dropouts and RockShox Tulio dropouts, which are much bigger and heavier than standard dropouts and therefore stronger, may be appropriate for the downhill and freeriding crowd. Forks, Hubs and Skewes designed for one system will not be compatible with any other. Whenever you put your wheels back on a bike, always check the quick release skewers are seated properly in the dropouts and done up tightly. If you ever spin a wheel and it rubs hard against a brake block, check that the skewer is properly seated in the dropout before adjusting the brakes. It's quite easy to get the standard ones in slightly squint.

the slots in the fork and rear triangle that the axle of the wheel mount to.

The hook-shaped pieces of the frame which contact the hub of the front and rear wheels. The dropouts attach the wheels to the bike.