Swash (uprush and backwash), in geography, is the water that washes up on shore after an incoming wave has broken. This action will cause sand and other light particles to be transported up the beach. The direction of the swash varies with the prevailing wind, whereas the backwash is always perpendicular to the coastline. This may cause longshore drift.
Backwash current is a seaward current that results from the receding swash on the beach face, after a wave breaks, joins the seaward movement of the wave trough toward the next incoming crest. The same orbital wave movement that causes a ball to bob up and down on the water causes the trough to move back and up toward the next wave crest. This is not what the word "undertow" suggests, and this term should not be used.