Multiplexer (mux)
Beginner
A switch that picks one of several inputs and passes it along.
Novice
A selector circuit: a control input decides which of several data inputs is passed to the output. An if-else in RTL usually becomes a multiplexer.
Expert
The hardware form of a conditional. Synthesis builds mux trees from if and case statements; resource sharing and flip-flop enables add muxes, and clock gating removes the enable muxes again.
Explained in Logic synthesis (Design Flow).