follicle

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A small bodily cavity or sac.
• noun A crypt or minute cul-de-sac or lacuna, such as the depression in the skin from which the hair emerges.
• noun An ovarian follicle.
• noun A spherical mass of cells usually containing a cavity.
• noun Botany A dry, single-chambered fruit that splits along only one seam to release its seeds, as in larkspur and milkweed.

from The Century Dictionary.

• noun In botany: A dry one-celled seed-vessel consisting of a single carpel, and dehiscent only by the ventral suture, as in the milkweed and larkspur.
• noun Any bladder-shaped appendage; a utricle.
• noun In anatomy and zoology, a minute secretory or excretory cavity, sac, or tube; one of the ultimate blind ramifications of a secretory surface; a glandular cul-de-sac; a mucous crypt or lacuna; a minute nodule of lymphoid tissue.
• noun In entomology, a cocoon; the covering made by a larva for its protection during the pupa state.

from the GNU version of the Collaborative International Dictionary of English.

• noun (Bot.) A simple podlike pericarp which contains several seeds and opens along the inner or ventral suture, as in the peony, larkspur and milkweed.
• noun A small cavity, tubular depression, or sac.
• noun A simple gland or glandular cavity; a crypt.
• noun A small mass of adenoid tissue.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun anatomy A small cavity or sac, such as a hair follicle.
• noun botany A type of primitive dry fruit produced by certain flowering plants.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun any small spherical group of cells containing a cavity