Short for TclExecuteByteCode, the C function that takes up most of generic/tclExecute.c, and (as the name suggests) is the main player when executing Tcl bytecode.
Famous for being an example of how to not write your code — TEBC currently looks like a bomb exploded in it ("don't look at it after lunch") — but hard to do in any other way; optimized bytecode executors usually are huge balls of spaghetti.