I. ˈplug-in 1 BrE AmE adjective [only before noun]
able to be connected to the electricity supply, or to another piece of electrical equipment:
a plug-in microphone
II. plug-in 2 BrE AmE , plug‧in /ˈplʌɡɪn/ noun [countable] technical
a piece of computer software that can be used in addition to existing software in order to make particular programs work properly