Any small container or space
compartment, cavity, receptacle; see container, hole 2.
A unit of a living organism
corpuscle, cellule, microorganism, spore, organism, egg, protoplasm, cytoplasm, vacuole, protoplast, embryo, germ, follicle, red blood cell, erythrocyte, white blood cell, leukocyte, hemocyte.
A small room
cubicle, vault, hold, pen, cage, hole, coop, dungeon, keep, chamber, den, recess, retreat, alcove, niche, crypt, crib, nook, burrow, stall, closet, booth, cloister, antechamber, compartment, number, lockup*; see also room 2.
A unit of an organization
group, block, claque, cadre; see faction 1, organization 3.