Thallus \Thal"lus\, n.; pl. Thalli. [NL., fr. Gr. ? young shoot or branch, frond.]

(Bot.) A solid mass of cellular tissue, consisting of one or more layers, usually in the form of a flat stratum or expansion, but sometimes erect or pendulous, and elongated and branching, and forming the substance of the thallogens.

Source: Webster's Revised Unabridged Dictionary (1913)