Gall: An abnormal growth or swelling in a plant. The formation of a gall may be induced by infection of the plant with bacteria or fungi, or by attack from certain mites, nematodes, or insects.*
The fuzzy, red ball on this rose stem is a moss gall. It formed when a gall wasp deposited her eggs on the plant. The wasps cause a chemical distortion in the plant tissue, resulting in the abnormal ‘mossy’ growth. The wasp larvae are protected inside the gall as they grow.
*Allaby, M. (2006). A dictionary of plant sciences (Revised ed.). Oxford: Oxford University Press.