(As in an aeroplane "cargo bay") A space in a cabinet into which a device of a certain size can be physically mounted and connected to power and data.

Common examples are a "drive bay" into which a disk drive (usually either 3.5 inch or 5.25 inch) can be inserted or the space in a docking station where you insert a notebook computer or laptop computer to work as a desktop computer or to charge their batteries, print or connect to the office network, etc.