The term for a word that is the opposite of another word is "antonym." Antonyms are pairs of words that express contrasting meanings, such as "hot" and "cold," "happy" and "sad," or "fast" and "slow." The concept of antonyms is fundamental in language, as it allows for nuanced communication and clearer expression of ideas. In contrast to antonyms, synonyms are words that have similar meanings. Understanding both antonyms and synonyms enhances vocabulary and aids in more precise language usage, facilitating effective communication.