From Glossary of Meteorology
A drop of water of diameter 0.2–0.5 mm (0.008–0.02 in.) falling usually (but not always) from low stratus or stratocumulus cloud.
Although this is the correct term for this size range, all water drops of diameter greater than 0.2 mm are frequently termed raindrops, as opposed to cloud drops. Should such drops reach the ground, they can be felt on the upturned face.