/ Dictionary / Index M main shaft: Meaning and Definition of Find definitions for: main' shaft" Pronunciation: [key] the principal shaft of a motor, transmission, etc. (distinguished from jackshaft). Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. main sequence mainsheet Related Content Daily Word Quiz: soporific Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language