Clock drift refers to several related phenomena where a clock does not run at exactly the same rate as a reference clock. That is, after some time the clock "drifts apart" or gradually desynchronizes from the other clock. All clocks are subject to drift, causing eventual divergence unless resynchronized. In particular, the drift of crystal-based clocks used in computers requires some synchronization mechanism for any high-speed communication. Computer clock drift can be utilized to build random number generators. These can however be exploited by timing attacks.

Everyday clocks such as wristwatches have finite precision. Eventually they require correction to remain accurate. The rate of drift depends on the clock's quality, sometimes the stability of the power source, the ambient temperature, and other subtle environmental variables. Thus the same clock can have different drift rates at different occasions.

More advanced clocks and old mechanical clocks often have some kind of speed trimmer where one can adjust the speed of the clock and thus correct for clock drift. For instance, in pendulum clocks the clock drift can be manipulated by slightly changing the length of the pendulum.

A quartz oscillator is less subject to drift due to manufacturing variances than the pendulum in a mechanical clock. Hence most everyday quartz clocks do not have an adjustable drift correction.

Atomic clocks are very precise and have nearly no clock drift. Even the Earth's rotation rate has more drift and variation in drift than an atomic clock due to tidal acceleration and other effects. The principle behind the atomic clock has enabled scientists to re-define the SI unit second in terms of exactly 9192631770 oscillations of the caesium-133 atom. The precision of these oscillations allows atomic clocks to drift roughly only one second in a hundred million years; as of 2015, the most accurate atomic clock loses one second every 15 billion years. The International Atomic Time (TAI) time standard and its derivatives (such as the Coordinated Universal Time (UTC)) are based on weighted averages of atomic clocks worldwide.

As Einstein predicted, relativistic effects can also cause clock drift due to time dilation. This is because there is no fixed universal time, time being relative to the observer. Special relativity describes how two clocks held by observers in different inertial frames (i.e. moving with respect to each other but not accelerating or decelerating) will each appear to either observer to tick at different rates.

In addition to this, general relativity gives us gravitational time dilation. Briefly, a clock in a stronger gravitational field (e.g. closer to a planet) will appear to tick more slowly. People holding these clocks (i.e. those inside and outside the stronger field) would all agree on which clocks appear to be going faster.

It is time itself rather than the function of the clock which is affected. Both effects have been experimentally observed.^{[citation needed]}