mid-13c., "flat sheet of gold or silver," also "flat, round coin," from Old French plate "thin piece of metal" (late 12c.), from Medieval Latin plata "plate, piece of metal," perhaps via Vulgar Latin *plattus, formed on model of Greek platys "flat, broad" (from PIE root *plat- "to spread"). The cognate in Spanish (plata) and Portuguese (prata) has become the usual word for "silver," superseding argento via shortening of *plata d'argento "plate of silver, coin."
Meaning "table utensils" (originally of silver or gold only) is from Middle English. Meaning "shallow dish for food," now usually of china or earthenware, originally of metal or wood, is from mid-15c. Baseball sense is from 1857. Geological sense is first attested 1904; plate tectonics is attested from 1967. Plate-glass first recorded 1727.