Dictionary of Audio Engineer Terms

FX = foreign exchange (money) first and foremost for me. Been around for ages longer than plugin effects.

Have to mention one of my favorite pro audio dictionaries, Rane Pro Audio Reference

Signal processing literature often speaks of "filters" when they mean a component or system that somehow changes the incoming signal. The change need not be one that acts like a filter that removes frequencies. It is of course very confusing to use the word "filter" with that meaning in general audio talk since there the meaning is immediately understood to mean a filter that acts on the frequency balance of a signal. Nevertheless, for example the Tracktion DAW used to call its effects "filters". Reaper uses the word FX to mean plugins that either change ("effect") the incoming signal or generate a signal (instruments). These plugins can be VST, DX, AU (on Mac), JS plugins. VSTi simply means "VST instrument" and is not a whole new plugin format. Usually the user interface controls in a plugin correspond to a "parameter", such as stereo width or filter frequency. Each parameter may or may not belong to a subcomponent in the plugin, but this is usually of no concern for the user. These parameters can also be presented in a DAW like Reaper in alternative ways, such as automation envelopes. (Other presentations include the generic slider-based GUI that can be turned on for plugins that otherwise have a fancy looking user interface or adding parameters to the Reaper track control panels as knobs.)