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 a number of different parts of speech, such as exclamations (ouchch!, 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.
This is a companion discussion topic for the original entry at http://grammarerror.com/tags/interjection