An interjection is a word or expression that occurs as an utterance on its own and expresses a spontaneous feeling or reaction. It is a diverse category, encompassing many different parts of speech, such as exclamations (ouch!, wow!), curses (damn!), greetings (hey, bye), response particles (okay, oh!, m-hm, huh?), hesitation markers (uh, er, um) and other words (stop, cool). Due to its diverse nature, the category of interjections partly overlaps with a few other categories like profanities, discourse markers and fillers. The use and linguistic discussion of interjections can be traced historically through the Greek and Latin Modistae over many centuries.

Greek and Latin intellectuals as well as the Modistae have contributed to the different perspectives of interjections in language throughout history.The Greeks held that interjections fell into the grammatical category of adverbs. They thought interjections modified the verb much in the same way as adverbs do, thus interjections were closely connected to verbs.

Unlike their Greek counterparts, many Latin scholars took the position that interjections did not rely on verbs and were used to communicate emotions and abstract ideas. They considered interjections to be their own independent part of speech. Further, the Latin grammarians classified any small non-word utterances as interjections.

Several hundred years later, the 13th- and 14th-century Modistae took inconsistent approaches to interjections. Some, such as Thomas of Erfurt, agreed with the former Greeks that the interjection was closely tied to the verb while others like Siger of Courtrai held that the interjection was its own part of speech syntactically, much like the Latin scholars.

In contrast to typical words and sentences, the function of most interjections is related to an expression of feeling, rather than representing some idea or concept.Generally, interjections can be classified into three types of meaning: volitive, emotive, or cognitive.

1. Volitive interjections function as imperative or directive expressions, requesting or demanding something from the addressee (e.g. Shh! = Be quiet!).
2. Emotive interjections are used to express emotions, such as disgust and fear (e.g. Yuck! = disgust).
3. Cognitive interjections express feelings which are more related to cognition, or information known to the speaker of the utterance (e.g. Wow! = surprise).

While there exists some apparent overlap between emotive and cognitive interjections, as both express a feeling, cognitive interjections can be seen as more related to knowledge of something (i.e. information previously known to the speaker, or recently learned).