Definition -
(1) A word-book or dictionary; especially a dictionary of Greek, Hebrew, Syriac, or Arabic.
(2) A vocabulary proper to a particular sphere of activity (e.g. a lexicon of love).
(3) The set of lexemes (i.e. words that are the source of multiple inflected forms) that make up a language.

Etymology -
The word derives from the Greek lexikon biblion, word book (from lexis, word).

Oxford English Dictionary -
The first citation for the word in its word-book sense is from 1603; the first citation for its specialized-vocabulary sense is from 1647:
"This barbarous Term you will not meet In all Love's Lexicon."
(Cowley Mistress, Discretion, 66)

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