Yes and no

noyesyes" or "noyes or noAyenegatorycolloquial form of yesNegative and positive particlesnopeuh-huh
Yes and no, or word pairs with a similar usage, are expressions of the affirmative and the negative, respectively, in several languages including English.wikipedia
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Yes

yeajaYeah
The words yes and no are not easily classified into any of the eight conventional parts of speech.
* An affirmative particle in the English language; see yes and no

No

nay
The words yes and no are not easily classified into any of the eight conventional parts of speech.
Yes and no are positive and negative responses

Affirmation and negation

negationnegativepolarity
Yes and no, or word pairs with a similar usage, are expressions of the affirmative and the negative, respectively, in several languages including English.
Affirmative and negative responses (especially, though not exclusively, to questions) are often expressed using particles or words such as yes and no, where yes is the affirmative and no the negative particle.

Pro-sentence

prosentence
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.
In English, yes, no, okay and amen are common pro-sentences.

Yes–no question

yes-no questionyes/no questionpolar question
Some languages do not answer yes–no questions with single words meaning 'yes' or 'no'. In all Sinitic/Chinese languages, yes-no questions are often posed in A-not-A form, and the replies to such questions are echo answers that echo either A or not A.
In linguistics, a yes–no question, formally known as a polar question or a general question, is a question whose expected answer is either "yes" or "no".

Sentence word

sentence wordsword sentenceword sentences
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.
Various phrases in various languages have devolved into the words for "yes" and "no" (which can be found discussed in detail in yes and no), and these include expletive sentence words such as "Well!"

Non

French has oui, si, and non.
* Non, a negatory word in French and Latin

Nay

While Modern English has a two-form system of yes and no for affirmatives and negatives, earlier forms of English had a four-form system, comprising the words yea, nay, yes, and no.
An archaic form of no, used mainly in oral voting, and the opposite of yea

Echo answer

echo responserepeats the verb used in the question
(This is an echo response.) The answer to "Tuletteko kaupungista?" In all Sinitic/Chinese languages, yes-no questions are often posed in A-not-A form, and the replies to such questions are echo answers that echo either A or not A.
In linguistics, an echo answer or echo response is a way of answering a polar question without using words for yes and no.

A-not-A question

A-not-A
In all Sinitic/Chinese languages, yes-no questions are often posed in A-not-A form, and the replies to such questions are echo answers that echo either A or not A.
is a neutral question where the answer to this can be yes or no in response to the first and more explicitly stated alternative.

Aye aye, sir

ayeAye, AyeAye-Aye
In naval language, the phrase "aye aye, sir" is used to acknowledge a direct order, and indicates the speaker both understands the order and will comply with it; a single "aye" is a synonym for "yes" and simply indicates agreement.
It is derived from a duplicate of the word "aye" which came into the English language in the late 16th century and early 17th century, meaning "Yes; even so.".

English language

EnglishEnglish-languageen
Yes and no, or word pairs with a similar usage, are expressions of the affirmative and the negative, respectively, in several languages including English.

Middle English

EnglishMiddlelate Middle English
English originally used a four-form system up to and including Early Middle English but Modern English has reduced this to a two-form system consisting of just 'yes' and 'no'.

Modern English

EnglishModern18th century
English originally used a four-form system up to and including Early Middle English but Modern English has reduced this to a two-form system consisting of just 'yes' and 'no'.

Welsh language

WelshWelsh-languageWelsh-speaking
Welsh and Finnish are among several languages that typically employ echo answers (repeating the verb with either an affirmative or negative form) rather than using words for 'yes' and 'no', though both languages do also have words broadly similar to 'yes' and 'no'.

Finnish language

FinnishFinnish-languagefi
Welsh and Finnish are among several languages that typically employ echo answers (repeating the verb with either an affirmative or negative form) rather than using words for 'yes' and 'no', though both languages do also have words broadly similar to 'yes' and 'no'.

Part of speech

parts of speechclosed classword class
The words yes and no are not easily classified into any of the eight conventional parts of speech.

Interjection

interjectionsexclamationexclamatory particle
Although sometimes classified as interjections, they do not qualify as such, and they are not adverbs.

Adverb

adverbsadv.abstract noun
Although sometimes classified as interjections, they do not qualify as such, and they are not adverbs.

Sentence (linguistics)

sentencesentencesdeclarative sentence
Sentences consisting solely of one of these two words are classified as minor sentences.

Otto Jespersen

JespersenJespersen, OttoOtto
This is the position of Otto Jespersen, who states that "'Yes' and 'No' [...] are to all intents and purposes sentences just as much as the most delicately balanced sentences ever uttered by Demosthenes or penned by Samuel Johnson."

Balanced sentence

This is the position of Otto Jespersen, who states that "'Yes' and 'No' [...] are to all intents and purposes sentences just as much as the most delicately balanced sentences ever uttered by Demosthenes or penned by Samuel Johnson."

Demosthenes

Démosthène Demosthenes Demosthenem
This is the position of Otto Jespersen, who states that "'Yes' and 'No' [...] are to all intents and purposes sentences just as much as the most delicately balanced sentences ever uttered by Demosthenes or penned by Samuel Johnson."

Samuel Johnson

JohnsonDr JohnsonDr. Johnson
This is the position of Otto Jespersen, who states that "'Yes' and 'No' [...] are to all intents and purposes sentences just as much as the most delicately balanced sentences ever uttered by Demosthenes or penned by Samuel Johnson."

Georg von der Gabelentz

Gabelentz
Georg von der Gabelentz, Henry Sweet, and Philipp Wegener have all written on the subject of sentence words.