Any paint that uses water as a medium. Paintings done with this medium are known as watercolors. An exception to this rule is water miscible oil paints, which employ water as their medium, but are actually oil paints.
When made opaque with white, watercolor is generally called gouache or bodycolor. Tempera is another exception.
Colors are usually applied and spread with brushes, but other tools can also used. The most common techniques for applying watercolor are called wet-on-dry and wet-on-wet, along with the dry brush techniques dry-on-dry and dry-on-wet. Colors can be removed while still wet, to various degrees by blotting.
Most watercolor painting is done on paper, but other absorbent grounds can also be employed. The papers most favored by those who paint with watercolor is white, very thick, with high rag content, and has some tooth.
"Watercolor" is the American spelling. "Watercolour" is the British spelling.