The type of erosion you’re asking about is abrasion (also called corrasion).

In abrasion, waves act like liquid sandpaper. They pick up fragments of rock, pebbles, and sand, then hurl or drag them against the base of cliffs, slowly grinding and smoothing the rock away. Over time this can undercut the cliff, weaken it, and eventually cause sections to collapse, helping to shape steep coastal cliffs and wave-cut platforms.

So, when cliffs are gradually ground down by fragments of rock and sand within waves, the process is called abrasion.