/ Dictionary / Index L law of definite composition: Meaning and Definition of Find definitions for: law' of def'inite composi'tion Pronunciation: [key] the statement that in a pure compound the elements are always combined in fixed proportions by weight. the law that either a proposition or its denial must be true. Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. law of cosines law of diminishing marginal utility Related Content Daily Word Quiz: labyrinth Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language