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.
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!)  Another distinction based on form is between interjections that are single words ('Oh!', 'Wow!') versus those that are phrases ('sup!' from 'What's up?', 'Excuse me!, 'Oh dear!').
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.
Interjections across languages
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.
Examples from English
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:
- Ahem [m], [?m], [m], or [h?m], ("attention!") may contain a glottal stop [?] or a [?] in any dialect of English; the glottal stop is common in American English, some British dialects, and in other languages, such as German.
- Gah [?æh], [:] ("Gah, there's nothing to do!") ends with [h], which does not occur with regular English words.
- Psst [ps:t] ("here!") is an entirely consonantal syllable, and its consonant cluster does not occur initially in regular English words.
- Shh [?::] ("quiet!") is another entirely consonantal syllable word.
- Tut-tut [? ?] ("shame..."), also spelled tsk-tsk, is made up entirely of clicks, which are an active part of regular speech in several African languages. This particular click is dental. (This also has the spelling pronunciation [t?t t?t].)
- Ugh [?x] ("disgusting!") ends with a velar fricative consonant, which is otherwise restricted to just a few regional dialects of English, though is common in languages like Spanish, German, Gaelic and Russian.
- Whew or phew [u], ?ju ("what a relief!"), also spelled shew, may start with a bilabial fricative, a sound pronounced with a strong puff of air through the lips. This sound is a common phoneme in such languages as Suki (a language of New Guinea) and Ewe and Logba (both spoken in Ghana and Togo).
- Yeah [jæ] ("yes") ends with the vowel [æ], or in some dialects the short vowel [?] or tensed , none of which are found at the end of any regular English words.
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- ^ Wharton, Tim (2003). "Interjections, language, and the `showing/saying' continuum". Pragmatics & Cognition. 11: 39-91. doi:10.1075/pc.11.1.04wha.