“homonymy: The relation between words whose forms are the same but whose meanings are different and cannot be connected: e.g. between pen ‘writing instrument’ and pen ‘enclosure.’ Homonyms are words related in this way.
Distinguished from polysemy in that the meaning of one homonym is not seen as deriving from that of the other: in that light, the words are different lexical units. Homonymy can also be distinguished from cases of conversion: e.g. that of either of the nouns ‘pen’ into a corresponding verb. Also from syncretism, which is between inflections of the same lexical unit. The term may be restricted further to homonymy, as in this example, both in the sounds of words and in their spelling: hence the more specific homograph, homophone.”
Excerpted from: Matthews, P.H., ed. The Oxford Concise Dictionary of Linguistics. New York: Oxford University Press, 2014.