Tide is an obsolete or archaic term for time, period or season. It survives in compounds such as Yuletide, eventide, shrovetide, Eastertide, noontide, etc.

A simple noun tide was used synonymously with time and as a unit of time with hour in Old English and Middle English, but this usage became obsolete in the 15th century and survives only in occasional poetic reference to "occasion" or "favourable, proper time". Contemporary English narrows the application of the word to the tides of the sea.

The term "tide" has cognates in other Germanic languages, including Dutch (tijd) and German (Zeit). In German, the word for the tides of the sea (Gezeiten) is based on the root Zeit, indicating a relationship between these two words that is no longer present to the same degree in English. The Anglo Saxon word 'tyd', from which the 'tide' is derived, means 'seasons' in a general sense.

When used on sundials the 'tides' were around three hours long, starting at 6am and ending at 6pm, the working day divided up into these four tides. Working people would know which 'tide' it was through the ringing of a church bell.

Tides (from low-German 'tiet' = 'time') are the rise and fall of sea levels caused by the combined effects of the gravitational forces exerted by the Moon and the Sun and the rotation of the Earth.

Most places in the ocean usually experience two high tides and two low tides each day (semi-diurnal tide), but some locations experience only one high and one low tide each day (diurnal tide). The times and amplitude of the tides at the coast are influenced by the alignment of the Sun and Moon, by the pattern of tides in the deep ocean and by the shape of the coastline and near-shore bathymetry (see Timing).

Tides vary on timescales ranging from hours to years due to numerous influences. To make accurate records, tide gauges at fixed stations measure the water level over time. Gauges ignore variations caused by waves with periods shorter than minutes. These data are compared to the reference (or datum) level usually called mean sea level.

While tides are usually the largest source of short-term sea-level fluctuations, sea levels are also subject to forces such as wind and barometric pressure changes, resulting in storm surges, especially in shallow seas and near coasts.

Time is the indefinite continued progress of existence and events that occur in apparently irreversible succession from the past through the present to the future. Time is a component quantity of many measurements used to sequence events, to compare the durations of events and the intervals between them, and to quantify rates of change of quantities in material reality or in the conscious experience. The temporal position of events with respect to the transitory present is continually changing; events happen, then are located further and further in the past. Time has been a major subject of religion, philosophy, and science, but defining it in a manner applicable to all fields of study without circularity has consistently eluded scholars. A simple definition states that "time is what clocks measure".

Time is one of the seven fundamental physical quantities in the International System of Units. Time is used to define other quantities — such as velocity — so defining time in terms of such quantities would result in circularity of definition. An operational definition of time, wherein one says that observing a certain number of repetitions of one or another standard cyclical event (such as the passage of a free-swinging pendulum) constitutes one standard unit such as the second, is highly useful in the conduct of both advanced experiments and everyday affairs of life. The operational definition leaves aside the question whether there is something called time, apart from the counting activity just mentioned, that flows and that can be measured. Investigations of a single continuum called spacetime bring questions about space into questions about time, questions that have their roots in the works of early students of natural philosophy.