coma

(astronomy) A cloud of dust surrounding the nucleus of a comet.

(optics) A defect characterized by diffuse, pear-shaped images that in an ideal image would appear as points.

(botany) A tuft or bunch, such as the assemblage of branches forming the head of a tree, a cluster of bracts when empty and terminating the inflorescence of a plant, or a tuft of long hairs on certain seeds.