Adapter may refer to any of the following:
1. In general, an adapter refers to any circuitry or device that enables one type of device to communicate with another.
In your computer, hardware adapters may be "onboard" (soldered directly onto the motherboard). Or, they may be "discrete" (separate hardware, attached to a port on the motherboard, so that they can be removed or replaced).
An example of a discrete adapter is a video card connected to your motherboard's PCI slot, which allows your motherboard to communicate with your monitor. A discrete adapter is sometimes also called an expansion card.
Both "adapter" and "adaptor" are correct spellings for the term, and either may be used to refer to the same type of device.
2. In regards to connectors, an adapter is hardware that "adapts" one connection type to another. Pictured at right is a PS/2 to USB adapter, which allows a PS/2 keyboard or mouse to connect to a USB port.
3. The term adapter may also refer to an AC adapter or power supply. These devices convert the electricity from a main power source, such as an outlet. The electricity is adapted to the voltage required by electronic devices, such as your desktop computer, laptop, or printer.
5. In computer software, adapter may refer to intermediate code that facilitates communication between two separate programs. For example, adapter software may allow a program written in the C programming language to be directly accessed by a program written in Python. This type of software is also called a wrapper.