/ Dictionary / Index S semidiameter: Meaning and Definition of Find definitions for: sem•i•di•am•e•ter Pronunciation: (sem"ē-dī-am'i-tur, sem"ī-), [key] — n. half of a diameter; radius. Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. See also: semidiameter (Thesaurus) semidetached semidiurnal Related Content Daily Word Quiz: fidelity Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language