Fis"sion (?), n. [L. fissio. See Fissure.] 1. A cleaving, splitting, or breaking up into parts.

2. (Biol.) A method of asexual reproduction among the lowest (unicellular) organisms by means of a process of self-division, consisting of gradual division or cleavage of the into two parts, each of which then becomes a separate and independent organisms; as when a cell in an animal or plant, or its germ, undergoes a spontaneous division, and the parts again subdivide. See Segmentation, and Cell division, under Division.

3. (Zoöl.) A process by which certain coral polyps, echinoderms, annelids, etc., spontaneously subdivide, each individual thus forming two or more new ones. See Strobilation.

• The process whereby one item splits to become two.
  
• (physics): The process of splitting an atom into smaller particles.
  
• (biology): The process by which a bacterium splits to form two daughter cells.