What is Adapter?

Legal Definition
An adapter or adaptor is a device that converts attributes of one electrical device or system to those of an otherwise incompatible device or system. Some modify power or signal attributes, while others merely adapt the physical form of one electrical connector to another.

An electric power adapter may enable connection of a power plug, sometimes called a travel plug, used in one region to a AC power socket used in another, by offering connections for the disparate contact arrangements, while not changing the voltage. An AC adapter, also called a "recharger", is a small power supply that changes household electric current from distribution voltage (in the range 100 to 240 volts AC) to low voltage DC suitable for consumer electronics.

For computers and related items, one kind of serial port adapter enables connections between 25-contact and nine-contact connectors, but does not affect electrical power- and signalling-related attributes.
-- Wikipedia
Legal Definition
Any device connecting interfaces allowing the to work together.