Una corriente de resaca es una fuerte corriente superficial (o casi superficial) de agua, que retrocede desde la costa hacia el mar. Se genera principalmente por el rompimiento irregular de las olas a lo largo de la cresta, llegando bruscamente a la playa con un índice elevado de energía, desvaneciéndose luego sobre el fondo para, posteriormente, regresar hacia el mar por un canal a través de las olas. Su intensidad depende de la altura de las olas y de las características topográficas de la orilla, siendo además reforzadas por las corrientes de marea, por lo que se hacen más peligrosas en bajamar. Estas corrientes pueden ser visibles o no dependiendo de la intensidad de la corriente y del tipo de sedimento que se encuentra en la playa.

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A rip current, often referred to simply as a rip, or by the misnomer "rip tide", is one specific kind of water current which can be found near beaches with breaking waves. Rip currents can endanger inexperienced swimmers. A rip is a strong, localized, and narrow current of water which moves directly away from the shore, cutting through the lines of breaking waves like a river running out to sea; a rip current is strongest near the surface of the water. Rip currents can occur at any beach where there are breaking waves: on oceans, seas, and large lakes. The location of rip currents can be unpredictable: while some tend to recur always in the same place, others can appear and disappear suddenly at various locations along the beach. A rip current forms because breaking waves push water towards the land. Water that has been pushed up near the beach flows together (as feeder currents), and this water finds a place where it can flow back out to sea. The water then flows out at a right angle to the beach in a tight current called the "neck" of the rip, where the flow is most rapid. When the water in the rip current reaches outside of the lines of breaking waves, the flow loses power, and dissipates in what is known as the "head" of the rip. Sometimes tendrils of left-over current then actually curve back towards the shore. Rip currents can be hazardous to people who are in the water. Swimmers or floaters who are caught in a rip and who do not understand what is going on, and who may also not have the necessary water skills, may panic, or may exhaust themselves by trying to swim directly against the flow of water. Because of these factors, rips are the leading cause of rescues by lifeguards at beaches, and in the US rips are responsible for an average of 46 deaths from drowning each year. A rip current is not the same thing as undertow, although some people use the latter term incorrectly when they mean a rip current. Contrary to popular belief, neither rip nor undertow can pull a person vertically down and hold them under the water surface; A rip simply carries floating objects, including people, to an area outside the zone of the breaking waves.

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