An interjection is a non-inflectable lexeme which has the distribution of a sentence and conveys the speaker's reaction to something in the speech situation or in the universe of discourse. The prototypical interjection is a particle, like Ouch! Secondarily, morphologically or even syntactically complex lexemes like Really!? or Good heavens! count as interjections.

Interjections classify by structural and semantic criteria:

This produces the following cross-classification:

structural
semantic  ╲
primarysecondary
deicticOuch!Good heavens!
anaphoricNo.Hear, hear!