Original: A rip current, commonly miscalled a rip tide, is a strong channel of water flowing seaward from near the shore, typically through the surf line.
Simplified: A rip current is a strong surface flow of water returning seaward from near the shore (not to be confused with an undertow). It is often mistakenly called a "rip tide" or "riptide", though the occurrence is not related to the tides.