From Beachapedia

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Accumulation of wave-deposited or stream-deposited loose sediment, usually sand, that extends from the outermost breakers to the landward limit of the wave and swash action.

For an in-depth understanding of how beaches form and change, watch the 20-minute video The Beach - A River of Sand (1965) which was made by Doug Inman, a marine geologist at Scripps Institution of Oceanography.