In linguistics, an interjection is a word or expression that occurs as an utterance on its own and expresses a spontaneous feeling or reaction. The category is quite heterogeneous, and includes such things as exclamations (ouch!, wow!), curses (damn!), greetings (hey, bye), response particles (okay, oh!, m-hm, huh?), and hesitation markers (uh, er, um). Due to its heterogeneous nature, the category of interjections partly overlaps with 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.
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).
Interjections may be subdivided and classified in several ways. A common distinction is based on relations to other word categories: primary interjections are interjections first and foremost (examples: Oops., Ouch!, Huh?), while secondary interjections are words from other categories that come to be used as interjections in virtue of their meaning (examples: Damn!, Hell!)  Primary interjections are generally considered to be single words (Oh!, Wow!). Secondary interjections can consist of multi-word phrases, or interjectional phrases, (examples: sup! from What's up?, Excuse me!, Oh dear!, Thank God!), but can also include single-word alarm words (Help!), swear and taboo words (Heavens!), and other words used to show emotion (Drats!). Although secondary interjections tend to interact more with the words around them, a characteristic of all interjections--whether primary or secondary--is that they can stand alone. For example, it is possible to utter an interjection like ouch! or bloody hell! on its own, whereas a different part of speech that may seem similar in function and length, such as the conjunction and, cannot be uttered alone (you can't just say and! independently in English).
Further distinctions can be made based on function. Exclamations and curses are primarily about giving expression to private feelings or emotions, while response particles and hesitation markers are primarily directed at managing the flow of social interaction.
Many consider interjections to fall under the part of speech of particles which also includes adverbs, fillers and onomatopoeias. As particles share many similarities, there exists confusion as to how to correctly define one subcategory from another. For example, one way to differentiate interjections from onomatopoeias is to take into account that interjections are not created by inanimate sources. That is, onomatopoeias can be created from sounds made by tools, machines, and non-animate sources in nature as well as animate sources such as animals. Thus, interjections are by and large created by animate sources which in almost all cases come from humans. 
Interjections are often confused as adverbs as they often appear following a form of the verb "go" causing many to think it is describing the verb instead of supplementing the utterance (example: One way to differentiate between an interjection and adverb is to find the locutor of the utterance. That is, if the locutor is making the utterance then it is an interjection. However, if the locutor is making a description of an utterance made by another locutor then it is considered an adverb. 
There is also a need to make a distinction between fillers and interjections. Often considered the same, interjections belong to their own subcategory in that they are able to stand freely outside of an utterance and may include pauses without losing its semantic meaning. Further, interjections are able to begin a direct and hypothetical utterance unlike fillers. Finally, many words of expressions considered as fillers are not classified as belonging to interjections. 
Routines are considered as a form of speech acts that rely on an understood social communicative pattern between the addressee and addressed. This differs from an interjection that is more of a strategic utterance within a speech act that brings attention to the utterance but may or may not also have an intended addressed (directed at an individual or group). In addition, routines generally are multi-word expressions whereas interjections tend to be single utterances.
Not all scholars agree that interjections can be grouped with particles. Felix Ameka maintains that the two are distinct in that particles cannot be independent utterances and are fully a part of the syntax of the utterance. Interjections, on the other hand, can stand alone and also are always preceded by a pause, separating them from the grammar and syntax of other surrounding utterances.
Interjections are bound by context, meaning that their interpretation is largely dependent on the time and place at which they are uttered. In linguistics, interjections can also be considered a form of deixis. Although their meaning is fixed (e.g. "Wow!" = surprised), there is also a referencing element which is tied to the situation. For example, the use of the interjection "Wow!" necessarily references some relation between the speaker and something that has just caused surprise to the speaker at the moment of the utterance. Without context, the listener would not know the referent of the expression (i.e. the source of the surprise). Similarly, the interjection "Ouch!" generally expresses pain, but also requires contextual information for the listener to determine the referent of the expression (i.e. the cause of the pain).
While we can often see deictic or indexical elements in expressive interjections, examples of reference are perhaps more clearly illustrated in the use of imperative examples. Volitive interjections such as "Ahem", "Psst!", and "Shh!" could be considered imperative, as the speaker is requesting or demanding something from the listener. Similar to the deictic pronoun "you", the referent of these expressions changes, dependent on the context of the utterance.
Interjections can take very different forms and meanings across cultures. For instance, the English interjections gee and wow have no direct equivalent in Polish, and the closest equivalent for Polish 'fu' (an interjection of disgust) is the different sounding 'Yuck!'. Curses likewise are famously language-specific and colourful. On the other hand, interjections that manage social interaction may be more similar across languages. For instance, the word 'Huh?', used when one has not caught what someone just said, is remarkably similar in 31 spoken languages around the world, prompting claims that it may be a universal word. Similar observations have been made for the interjections 'Oh!' (meaning, roughly, "now I see") and 'Mm/m-hm' (with the meaning "keep talking, I'm with you").
Across languages, interjections often use special sounds and syllable types that are not commonly used in other parts of the vocabulary. For instance, interjections like 'brr' and 'shh!' are made entirely of consonants, where in virtually all languages, words have to feature at least one vowel-like element. Some, like 'tut-tut' and 'ahem', are written like normal words, but their actual production involves clicks or throat-clearing. The phonetic atypicality of some interjections is one reason they have traditionally been considered as lying outside the realm of language.
Several English interjections contain sounds, or are sounds as opposed to words, that do not (or very rarely) exist in regular English phonological inventory. For example: