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. Atoms in a liquid slide over one another, allowing the liquid to flow.
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.