1. Botany, a very minute barb or beard: Samdy had to be very careful of the little cactus she had in her room, because the barbules were so tiny and minuscule and could be very painful if touched.
2. One of the processes along the edges of the barbs of a feather, by which adjacent barbs interlock: A barbule is a small barb or pointed projection, especially one of the small projections fringing the edges of the barbs of feathers.