Polysemy (/pəˈlɪsɪmi/ or /ˈpɒlɪˌsiːmi/; from Ancient Greek πολύ- (polý-) 'many' and σῆμα (sêma) 'sign') is the capacity for a sign (e.g. a symbol, morpheme, word, or phrase) to have multiple related meanings. For example, a word can have several word senses. Polysemy is the opposite of monosemy, which denotes a word with a single meaning.

Polysemy is distinct from homonymy—or homophony—which is an accidental similarity between two or more words (such as bear the animal, and the verb bear) : while homonymy is a mere linguistic coincidence, polysemy is not. In discerning whether a given set of meanings represent polysemy or homonymy, it is often necessary to look at the history of the word to see whether the two meanings are historically related. Dictionary writers often list polysemes (words or phrases with different, but related, senses) in the same entry (that is, under the same headword) and enter homonyms as separate headwords (usually with a numbering convention such as ¹bear and ²bear).

A polyseme is a word or phrase with different, but related, senses. Since the test for polysemy is the vague concept of the relatedness, judgments of polysemy can be difficult to make. Because applying pre-existing words to new situations is a natural process of language change, looking at words' etymology is helpful in determining polysemy but not the only solution; as words become lost in etymology, what once was a useful distinction of meaning may no longer be so. Some seemingly unrelated words share a common historical origin, however, so etymology is not an infallible test for polysemy, and dictionary writers also often defer to speakers' intuitions to judge polysemy in cases where it contradicts etymology. English has many polysemous words. For example, the verb "to get" can mean "procure" (I'll get the drinks), "become" (she got scared), "understand" (I get it) etc.

In linear or vertical polysemy, one sense of a word is a subset of the other. These are examples of hyponymy and hypernymy, and are sometimes called autohyponyms. For example, 'dog' can be used for 'male dog'. Alan Cruse identifies four types of linear polysemy:

In non-linear polysemy, the original sense of a word is used figuratively to provide a different way of looking at the new subject. Alan Cruse identifies three types of non-linear polysemy:

There are several tests for polysemy, but one of them is zeugma: if one word seems to exhibit zeugma when applied in different contexts, it is probable that the contexts bring out different polysemes of the same word. If the two senses of the same word do not seem to fit, yet seem related, then it is probable that they are polysemous. This test again depends on speakers' judgments about relatedness, which means that it is not infallible, but merely a helpful conceptual aid.

The difference between homonyms and polysemes is subtle. Lexicographers define polysemes within a single dictionary lemma, while homonyms are treated in separate entries, numbering different meanings (or lemmata). Semantic shift can separate a polysemous word into separate homonyms. For example, check as in "bank check" (or Cheque), check in chess, and check meaning "verification" are considered homonyms, while they originated as a single word derived from chess in the 14th century. Psycholinguistic experiments have shown that homonyms and polysemes are represented differently within people's mental lexicon: while the different meanings of homonyms (which are semantically unrelated) tend to interfere or compete with each other during comprehension, this does not usually occur for the polysemes that have semantically related meanings. Results for this contention, however, have been mixed.

For Dick Hebdige, polysemy means that, "each text is seen to generate a potentially infinite range of meanings," making, according to Richard Middleton, "any homology, out of the most heterogeneous materials, possible. The idea of signifying practice—texts not as communicating or expressing a pre-existing meaning but as 'positioning subjects' within a process of semiosis—changes the whole basis of creating social meaning".