Yes and no
From The Art and Popular Culture Encyclopedia
Yes and no are two words for expressing affirmatives and negatives respectively in English ("Are you hungry?" "Yes, I am."). Early Middle English had a four-form system, but Modern English has reduced this to a two-form system consisting of yes and no. Some languages do not answer yes-no questions with single words meaning 'yes' or no. At least one, Welsh, is widely but erroneously believed to have no words for yes and no. In fact it has, these being but one of the many ways in which yes-no questions are answered in Welsh. Welsh and Finnish are among several languages that employ echo answers rather than words for yes and no in such circumstances. Other languages have systems named two-form, three-form, and four-form systems, depending on how many words for yes and no they employ. Some languages, such as Latin, have no yes-no word systems.
The words yes and no are not easily classified into any of the eight conventional parts of speech. Although sometimes classified as interjections, they do not qualify as such, and they are not adverbs. They are sometimes classified as a part of speech in their own right, sentence words, word sentences, or pro-sentences, although that category contains more than yes and no and not all linguists include them in their lists of sentence words. Sentences consisting solely of one of these two words are classified as minor sentences.
The differences among languages, the fact that in different languages the various words for yes and no have different parts of speech and different usages, and that some languages lack a 'yes-no' word system, makes idiomatic translation difficult.