* Oxymoron: A figure of speech that combines two seemingly contradictory or opposite words, like "living dead" or "deafening silence." This is the closest concept, but it typically involves two words, not one.
* Paradox: A statement that seems contradictory or absurd, but may be true. It often requires further thought and interpretation. While a single word can be paradoxical in meaning, this is usually more about its context than a specific feature of the word itself.
* Antithesis: A rhetorical device that uses contrasting ideas or words to create a balance or emphasis. This is more about the juxtaposition of ideas, not necessarily individual words.
* Double entendre: A word or phrase with two meanings, often one literal and one metaphorical. This doesn't directly involve opposite meanings, but it can create a sense of ambiguity and irony within a poem.
Ultimately, the specific term you use would depend on the context of the word in the poem and the effect you want to achieve. You could simply describe the word as having "opposite meanings" or "contrasting connotations" within the poem.