From Glossary of Meteorology
A spherical particle of liquid water, from a few micrometers to a few tens of micrometers diameter, formed by condensation of water vapor on a hygroscopic aerosol particle (cloud condensation nucleus).
Such drops, apparently suspended in the air with other drops, form a visible cloud. Clouds may also contain interstitial haze particles, smaller than a few micrometers (μm) in diameter. Activation distinguishes a cloud from a haze, which contains only or mainly unactivated droplets. Cloud drops differ in size from drizzle drops and raindrops. A diameter of 0.2 mm has been suggested as an upper limit to the size of drops that shall be regarded as cloud drops; larger drops fall rapidly enough so that only very strong updrafts can sustain them. Any such division is somewhat arbitrary, and active cumulus clouds sometimes contain cloud drops much larger than this.