/ Dictionary / Index T thallus: Meaning and Definition of Find definitions for: thal•lus Pronunciation: (thal'us), [key] — pl. thal•li thal•lus•es. a simple vegetative body undifferentiated into true leaves, stem, and root, ranging from an aggregation of filaments to a complex plantlike form. Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. See also: thallus (Thesaurus) thallous thalweg Related Content Daily Word Quiz: impetuous Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language