Adapter is also known as wrapper pattern.
Adapter converts interface of a class into another interface a client expects. Adapter lets classes work together that couldn't otherwise because of incompatible interfaces.
Major point is that Adapter does not implement new functionality, it does not extend interface.
Pattern type - Structural
Adapter does not implement new functionality but allows to handle new interfaces/different without reworking clients.
This pattern is very useful in a situation when interface changes and for some reason we cannot modify client e.g. we have some legacy code (or vice versa).
Imagine HDMI to DVI converter (very real case shown in software domain).
IHDMI defines an interface for HDMI device, similarly IDVI.
To allow communication between HDMI and DVI devices HDMItoDVI adapter is needed.
HDMIMonitorClient represents client and adaptee is DVIComputer.