means The regular rise and fall of sea level caused by the gravitational pull of the moon and sun on the Earth's oceans.

from From Old English 'tīd,' which simply meant 'time' or 'a season' — a sense that survives in words like 'Yuletide' and 'Eastertide,' and in the proverb 'time and tide wait for no man,' where the two words once meant nearly the same thing. It's related to Old High German 'zīt' and shares an ancient Germanic root tied to the idea of 'dividing' time into portions. The watery 'rise and fall of the sea' meaning is the newcomer, emerging in late Middle English — the sea's twice-daily rhythm being, after all, a kind of clock.