If you understand the meaning of etymon and lemma in the two following statements, you may want to skip this post:
Papyrus is the etymon of paper.
An etymological dictionary gives the etymons or etymologies of its lemmas.
An etymon is the antecedent form of a word. It’s the word or any of the separate words from which another word has developed historically by borrowing, derivation, compounding, or in any other way. For example, the etymon of the English word etymon is the Greek word etymos, “true.” Etymos in turn is the etymon of Greek étumon, “the true sense of a word according to its origin.”
In modern usage, etymology refers to the branch of linguistics that deals with determining the origin of words and the historical development of their form and meanings.
For the ancients, the etymology or “true meaning” of a word might be found in its shape, sound, or superficial resemblance to another word. For example, one Christian commentator (writing in Latin) explained the “true” meaning of the Latin word for death (mors) by connecting it with the Latin word for bite (morsus). According to the biblical account, death entered the world when Eve took a bite out of the forbidden fruit. Ergo, the “true” meaning of death is rooted in the disobedient act described in Genesis.
Modern lexicographers determine the origin of a word by looking for evidence based on sound shifts, spellings, and cognates between languages.
The plural of etymon is either etymons or etyma.
Lemma derives from a Greek verb meaning “to take.” The first definition given in the OED is in the context of mathematics: “proposition assumed or demonstrated which is subsidiary to some other.”
In the context of literature, a lemma is defined as “the argument or subject of a literary composition, prefixed as a heading or title; also, a motto appended to a picture.”
The first OED citation of lemma in the context of lexicography is dated 1951. The definition for this use of lemma is “a lexical item as it is presented, usually, in a standardized form, in a dictionary entry.”
If you look up a word in the online Merriam-Webster Unabridged, you will find the word printed in large red letters. That large red word is the lemma.
The plural of lemma is either lemmas or lemmata.
Another word for lemma in the context of a dictionary entry is definiendum. Like the lemma, the definiendum is the word that heads the entry in a dictionary.