granule

Short definition from Wiktionary

Noun

1. 1
   A tiny grain, a small particle.
2. 2
   A small structure in a cell.
3. 3
   A particle from 2 to 4 mm in diameter, following the Wentworth scale
4. 4
   a small mark in the photosphere of the sun caused by convection currents. See also Wikipedia:Granule (solar physics).