A keyway is the shaped channel in a lock cylinder

Cylinder lock

A cylinder lock is a lock constructed with a cylinder that a locksmith can easily unscrew to facilitate rekeying. The cylinder may contain any of a variety of locking mechanisms, including the pin tumbler lock, the wafer tumbler lock and the disc tumbler lock....

 into which the key slides to gain access to the lock tumblers. Lock keyway shapes vary widely with lock manufacturer, and many manufacturers have a number of unique profiles requiring a specifically milled key blank

Key blank

A key blank is a key that has not been cut to a specific bitting. The blank has a specific cross-sectional profile to match the keyway in a corresponding lock cylinder. Key blanks can be stamped with a manufacturer name, end-user logo or with a phrase, the most commonly seen being 'Do not duplicate'...

 to engage the lock's tumblers

Pin tumbler lock

The pin tumbler lock is a lock mechanism that uses pins of varying lengths to prevent the lock from opening without the correct key...

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Background

Most people refer to lock keyways as keyholes, a linguistic carryover from the days when locks utilized bit keys that actually entered the lock through a hole in the door.