noun

1. a visible mass of tiny, condensed water droplets or ice crystals suspended in the atmosphere: clouds are commonly classified in four groups: A (high clouds above 6,096 m or 20,000 ft) cirrus, cirrostratus, cirrocumulus; B (intermediate clouds, 1,981 m to 6,096 m or 6,500 to 20,000 ft) altostratus, altocumulus; C (low clouds, below 1,981 m or 6,500 ft) stratus, stratocumulus, nimbostratus; D (clouds of great vertical continuity) cumulus, cumulonimbus
2. a mass of smoke, dust, steam, etc.
3. a great number of things close together and in motion a cloud of locusts
4. an appearance of murkiness or dimness, as in a liquid
5. a dark marking, as in marble
6. anything that darkens, obscures, threatens, or makes gloomy

Etymology: ME cloude, clude, orig., mass of rock, hence, mass of cloud < OE clud, mass of rock: for IE base see climb

transitive verb

1. to cover or make dark as with clouds
2. to make muddy or foggy
3. to darken; obscure; threaten
4. to make gloomy or troubled
5. to cast slurs on; sully (a reputation, etc.)

intransitive verb

1. to become cloudy
2. to become gloomy or troubled