ARE YOU GUYS KIDDING ME!!!???? UNDER-TOW? This term is not even used by my grandmother anymore...Learn your ocean if your going to talk about it.
Undertow is a term used to describe the phenomenon when a wave washes up a steep beach, then quickly back down, and another wave crashes in over the top of the receding wave. The "undertow" it can pull little kids off their feet and suck them under (for a very short time). It's a rel phenomenon, completely different from a rip current, which is water escaping outward from the beach through slots in a sand bar.