means A small pocket of seawater left trapped among rocks when the tide goes out, sheltering anemones, crabs, snails, and other creatures until the water returns.

from A plain English compound of "tide" and "pool." "Tide" comes from Old English "tīd," meaning a period or season of time — the same root as "tidings" and the "-tide" in Yuletide — which over centuries narrowed to mean the timed rising and falling of the sea. "Pool" descends from Old English "pōl," a small body of standing water, with cousins in Dutch "poel" and German "Pfuhl." Put together, the word names exactly what it is: the puddle the timekeeping sea leaves behind.