/ Dictionary / Index M machine bolt: Meaning and Definition of Find definitions for: machine' bolt" Pronunciation: [key] a threaded fastener, used with a nut for connecting metal parts, having a thread diameter of about in. (6.4 mm) or more and a square or hexagonal head for tightening by a wrench. Cf.See illus. under. Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. machine machine gun Related Content Daily Word Quiz: profiterole Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language