VST stands for Virtual Studio Technology and is used to refer to both plugin (Virtual FX) and virtual synths although they are more correctly VSTi (the i standing for instrument)
The VST has become a standard of sorts and was created by Steinberg (Cubase)
I'm sure someone will give a more accurate in-depth explanation shortly* but it gets used to generically mean a virtual effect or synth.
*Edit. Someone already has.