An interjection is a non-mimetic holophrastic particle.

This entails that it is a non-inflecting word which has the distribution of a sentence but is not an ideophone. An interjection conveys the speaker's reaction to something in the speech situation or in the universe of discourse. The prototypical interjection is monomorphematic, 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!