An adapter or adaptor (both spellings in use in UK and US English) is a device used to match the physical or electrical characteristics of two different things so that a connection may be made between them.
An adapter may be very simple, connecting one kind of plug to another kind of socket, but not changing what passes through. One example would be a garden hose, which normally has threads that screw on, but can be attached to an adapter which allows it to snap easily on and off of faucets, other hoses, or attachments. Likewise, a serial port may require an adapter to connect between the old DB-25 type and the later DE-9 type, which have 25 and 9 pins (respectively) but only 9 wires each.
Other adapters may include a device that changes what is passing through, such as a transformer that adapts household electric current from high voltage (100 to 240 volts AC) to low voltage suitable for consumer electronics.