Pro-sentence

A pro-sentence is a function word or expression that substitutes for a whole sentence whose content is recoverable from the context. A pro-sentence is a kind of pro-form and is therefore anaphoric.

In English, yes, no, okay and amen are common pro-sentences. In response to the question "Does Mars have two moons?", the sentence "Yes" can be understood to abbreviate "Mars has two moons."

Pro-sentences are sometimes seen as grammatical interjections, since they are capable of very limited syntactical relations. But they can also be classified as a distinct part of speech, given that (other) interjections have meanings of their own and are often described as expressions of feelings or emotions.

Yes and no

In some languages, the equivalents to yes and no may substitute not only a whole sentence, but also a part of it, either the subject and the verb, or the verb and a complement, and can also constitute a subordinate clause.

The Portuguese word sim (yes) gives a good example:

In some languages, such as English, yes rebuts a negative question, whereas no affirms it. However, in Japanese, the equivalents of no (iie, uun, (i)ya) rebut a negative question, whereas the equivalents of yes (hai, ee, un) affirm it.