Strictly speaking, a software "sub-program" that runs by being loaded into another program. In the electronic music context, a plug-in is usually a soft synth or effect processor that loads and runs in the context of a host application, usually a sequencer or DAW application. The host provides access to the computer's user interfacs, and the necessary linkage for the plug-in to send and receive MIDI and audio. There are a number of standards for how this linkage works; these include VST (the most popular), TDM (a standard used by ProTools), and Audio Units (a standard built into Apple's OSX). A given plug-in has to be written to work with the chosen standard, but many plug-ins on the market are available in different versions for different standards. When selecting or purchasing plug-ins, the performer must be aware of which plug-in standards his sequencer or DAW software supports.