Purdue University's Department of Chemistry defines "liquid" as a state of matter that flows, can change its shape, is not easily compressible and maintains a relatively fixed volume. Liquid is one of the four primary states of matter, with the others being solid, gas and plasma.
Liquids consist of small particles that are close together with no regular arrangement. These particles vibrate, move about and slide past each other. As the temperature of a liquid increases, the increased vibrations of the particles cause distances between the particles to increase. When a liquid reaches its boiling point, the cohesive forces that bind the particles closely together break, and the liquid changes to its gaseous state. If the temperature is decreased, the distances between the particles become smaller. When the liquid reaches its freezing point, the particles will usually lock into a very specific order, called crystallizing, and the bonds between them become more rigid, changing the liquid into its solid state.