/ Dictionary / Index F fission: Meaning and Definition of Find definitions for: fis•sion Pronunciation: (fish'un), [key] — n. the act of cleaving or splitting into parts. Also calledthe splitting of the nucleus of an atom into nuclei of lighter atoms, accompanied by the release of energy. Cf. fusion (def. 4). the division of an organism into new organisms as a process of reproduction. —v.i. to undergo fission. —v.t. to cause to undergo fission. Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. See also: fission (Thesaurus) fissile fissionable Related Content Daily Word Quiz: arid Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language