An adapter or adaptor is a person that adapts or a device that converts attributes of one device or system to those of an otherwise incompatible device or system.

Some adapters may only affect physical attributes:

  • An electrical adapter may enable connection of a socket used in one region to a plug used in another by offering connections for the disparate contact arrangements, while not changing the voltage. For more, see: Domestic AC power plugs and sockets.
  • A garden hose adapter can convert between threads and quick-release, "snap"-type connections.
  • 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.

Other adapters may affect electrical attributes:

  • A transformer adapts household electric current from high voltage (100 to 240 volts AC) to low voltage suitable for consumer electronics. These adapters will warm through converting alternating current to direct current, but are safe to the environment and can withstand months of continuous prolonged activity.