Liquid

Liquids are one of the states of matter. Specifically a dense state that usually is fairly incompressible (which means it doesn't change volume when pressure increases, as opposed to gases which are generally compressible). Liquids have a density fairly close to that of the solid state of that material (within a few percent). Liquids are fluids, which means that they flow easily.

While most materials will turn into a gas if they get to a high enough temperature, many materials will skip the liquid phase entirely unless under sufficient pressure. For example, carbon dioxide goes straight from solid to gas form unless under a fair amount of pressure (this is why frozen carbon dioxide is called 'dry ice'). At sufficiently high temperatures and pressures, the differences between gases and liquids disappear to form a supercritical fluid.

The study of liquids includes a fair amount of fluid mechanics, which is relevant for both tidal power and hydroelectricity. To learn more about liquids please see UC Davis's Chem wiki.