Short for TclExecuteByteCode, is the guts of the Tcl virtual machine and 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.
DKF: Note that you can't call this function directly, even from within the rest of Tcl. That's what Tcl_Eval (and friends) are for.