DirectX plugin (DX/DXi)
A Windows-specific format for audio effects and instruments from the late 1990s that lost out to VST.
When PC audio editing took off in the late 1990s, Cakewalk and Sonic Foundry, makers of Sound Forge, built their effects on Microsoft's DirectShow architecture rather than Steinberg's VST. The upside was that an effect registered with the system appeared in every DirectX-aware application. Cakewalk later added DXi, a variant for virtual instruments, in the early 2000s.
The format was tied to Windows and to Microsoft's COM model, and developers who wanted to reach both Mac and PC chose VST. Once Cakewalk's Sonar gained VST support, there was little reason left to write DX. Today the name mostly turns up when someone opens an old project in search of its plugins — and fails to find them on a 64-bit system.