JSFX
REAPER's format for audio effects written as plain text files in the EEL2 language and compiled on load.
The name comes from Jesusonic, an effects engine Justin Frankel built in the mid-2000s that later moved into REAPER. A JSFX is an ordinary text file with sections for initialisation, sliders and the per-sample loop. When the effect loads, the code is translated straight to machine code, which makes it fast enough for real time even though it never passes through a conventional compiler.
The upshot is that every effect bundled with REAPER ships as readable source. Anyone wondering how a compressor or filter is put together can open it, change a line and hear the result without stopping playback. With a formal plugin format like VST or CLAP, the same experiment takes a compiler, an SDK and an afternoon.