[thal-uh s]
noun, plural thal·li [thal-ahy] /ˈθæl aɪ/, thal·lus·es. Botany, Mycology.
  1. a simple vegetative body undifferentiated into true leaves, stem, and root, ranging from an aggregation of filaments to a complex plantlike form.

Origin of thallus

1820–30; < New Latin < Greek thallós young shoot, twig Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018

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Historical Examples of thallus

British Dictionary definitions for thallus


noun plural thalli (ˈθælaɪ) or thalluses
  1. the undifferentiated vegetative body of algae, fungi, and lichens
Derived Formsthalloid, adjective

Word Origin for thallus

C19: from Latin, from Greek thallos green shoot, from thallein to bloom
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012

Word Origin and History for thallus

Latin, from Greek thallos "green shoot, twig," related to thalia "abundance," thalos "scion, child," ultimately from PIE root *dhal- "to bloom" (cf. Old Irish duilesc, a type of algae).

Online Etymology Dictionary, © 2010 Douglas Harper

thallus in Medicine


n. pl. thal•lus•es
  1. A plant body or fungus undifferentiated into stem, root, or leaf.
The American Heritage® Stedman's Medical Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.

thallus in Science


Plural thalli (thălī)
  1. A type of body found among plants and fungi that is not differentiated into roots, stems, or leaves. Thalli are found among lichens, mosses, liverworts, and many algae, as well as the gametophyte generations of horsetails and ferns, which have rhizoids but not true roots.
The American Heritage® Science Dictionary Copyright © 2011. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.