From AMS Glossary
Applied to any time-varying physical quantity, usually periodic, its displacement in time relative to a similar reference quantity.
For example, the temperature in soil depends on depth and time. If the surface temperature varies sinusoidally, so does the temperature at any depth, but it is out of phase (is delayed) relative to the surface temperature. There is a phase shift or phase delay or phase lag of soil temperature relative to surface temperature. Although these three terms are most often applied to waves, nothing inherent in the concept of a delay restricts it to waves.