the.com/antonym

the word that exists solely to disagree with another word

means a word that means the opposite of another word, like 'hot' to 'cold' or 'up' to 'down'

from From Greek 'anti-' (against, opposite) plus '-onym' (name, word), so literally an 'opposite-name.' That '-onym' is the same word-part you meet in synonym, homonym, and pseudonym — a whole family descended from Greek 'onoma,' meaning name. 'Antonym' itself is a relatively late, deliberate coinage, built by 19th-century lexicographers as a tidy mirror-image of the older 'synonym.'

greek roots: means against-name, language built for opposition
not always neat: hot opposes cold, but warm crashes the binary
three flavors: gradable, complementary, and relational opposites exist
auto-antonyms: some words like cleave are their own antonym