What term describes two words that express opposing concepts?

The term that describes two words that express opposing concepts is "antonym." Antonyms are pairs of words that have contrasting meanings, such as "hot" and "cold" or "happy" and "sad." This relationship is fundamental in language, as it helps to convey differences in meaning and allows for more nuanced communication. By understanding antonyms, one can grasp the full scope of language and the ways in which words can relate to one another through opposition.

The other terms relate to different concepts: synonyms are words with similar meanings, homonyms are words that sound alike but have different meanings, and adverbs modify verbs, adjectives, or other adverbs and do not specifically relate to opposing concepts. This makes "antonym" the clear and correct choice in the context of the question.