A cloud was a visible mass of droplets or frozen crystals floating in the atmosphere above the surface of a planet or other astronomical body. The most common form of clouds on M-classplanets were made of water vapor. Low-level clouds very near or at a planet's surface were called fog.
A cloud could also refer informally to a concentration of matter drawn together by gravity. For example, nebulae were sometimes known as interstellar dust clouds. (TOS: "The Immunity Syndrome")
A common consequence of water vapor clouds on planets is the meteorological phenomenon known as rain. Storms, lightning, and thunder are also associated with clouds.