A bus (one "s") is any point at which two or more signals are combined into one signal. Aux is short for auxiliary and refers to any bus besides the main mix bus. A send is where signal is directed out from some point. A return is where a signal is brought back into a device (usually a mixing board).
"Bus" derives from Latin, as in omnibus. It comes from the same root as the kind of bus you ride and shares the "collective transportation" meaning, in the one case of people and in the other of signals. Technically "omni" is the part that means "all" and "ibus" is just a word ending, but that's how it worked out. "Buss" is a fuse company.