Definition - The capacity for a word or phrase to have multiple meanings.

Example -
Look at the size of those cranes!
(The word cranes is polysemous because it could denote either a bird or a gantry.)

Etymology -
The word derives from the Greek polusemos, having many meanings (from poly, many + sema, sign).

Oxford English Dictionary -
Its first citation is from 1900: "The new meaning of a word, whatever it may be, does not make an end of the old. They exist alongside of one another…. In proportion as a new signification is given to a word, it appears to multiply and produce fresh examples, similar in form, but differing in value. We shall call this phenomenon of multiplication Polysemia."
(N. Cust tr. Semantics xiv. 140)

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