What defines homonyms?

Homonyms are defined as words that sound and are spelled alike but have different meanings. This characteristic is essential in understanding how homonyms function in language. For instance, the word "bat" can refer to a flying mammal or a piece of sports equipment used in baseball. Despite their identical spelling and pronunciation, the meanings are distinct, showcasing the complexity and richness of language.

The other choices do not accurately represent homonyms. Some words that sound alike but differ in spelling would be classified as homophones, which is a different category of wordplay entirely. Synonyms are words with similar meanings, thus not related to the concept of homonyms. The idea that homonyms could only be nouns is also incorrect, as they can encompass verbs and adjectives as well. Therefore, the definition highlighted in the first choice precisely captures the essence of what homonyms are.