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Old English tid "point or portion of time, due time," from Proto-Germanic *tidiz "division of time" (cf. Old Saxon tid, Dutch tijd, Old High German zit, German Zeit "time"), from PIE *di-ti- "division, division of time," suffixed form of root *da- "to divide, cut up" (cf. Sanskrit dati "cuts, divides;" Greek demos "people, land," perhaps literally "division of society;" daiesthai "to divide;" Old Irish dam "troop, company").
Meaning "rise and fall of the sea" (mid-14c.) is probably via notion of "fixed time," specifically "time of high water;" either a native evolution or from Middle Low German getide (cf. also Dutch tij, German Gezeiten "flood tide"). Old English seems to have had no specific word for this, using flod and ebba to refer to the rise and fall. Old English heahtid "high tide" meant "festival, high day."
"to carry (as the tide does)," 1620s, from tide (n.). Usually with over. Related: Tided; tiding.
An alternate increase and decrease, as of levels of a substance in the blood or digestive tract.
The regular rise and fall in the surface level of the Earth's oceans, seas, and bays caused by the gravitational attraction of the Moon and to a lesser extent of the Sun. The maximum high tides (or spring tides) occur when the Moon and Sun are directly aligned with Earth, so that their gravitational pull on Earth's waters is along the same line and is reinforced. The lowest high tides (or neap tides) occur when the Moon and Sun are at right angles to each other, so that their gravitational pull on Earth's waters originates from two different directions and is mitigated. Tides vary greatly by region and are influenced by sea-floor topography, storms, and water currents. See also ebb tide, flood tide, neap tide, spring tide.