One of the four phases of matter, a liquid is a fluid whose volume is fixed under conditions of constant temperature and pressure; and, whose shape is usually determined by the container it fills.

Liquids tend to pull themselves together into droplets due to surface tension. The kinetic energy in a liquid is greater than that of a solid, but less than that of a gas. A liquids atoms/molecules are "slippery"; that is, they slide over one another, allowing the liquid to flow.

If a liquid is at rest, the pressure at any point is given by

where is the density of the liquid and is the depth of the point below the surface. Note that this formula assumes that the pressure at the free surface is zero.

It should be noted that glass at normal temperatures is not a "supercooled liquid", but a solid. See the article on glass for more details.

See also: gas, solid, plasma, ripple.

In linguistics, "liquid" can also refer to a liquid consonant.