interjection

n. interjecting of a word or phrase in a conversation; word or phrase inserted in a conversation; exclamation expressing emotion or feeling (Grammar)

An interjection is a part of speech that usually has no grammatical connection to the rest of the sentence and simply expresses emotion on the part of the speaker, although most interjections have clear definitions.  Filled pauses such as uh, er, um, are also considered interjections. Interjections are generally uninflected function words and have sometimes been seen as sentence-words, since they can replace or be replaced by a whole sentence (they are holophrastic). Sometimes, however, interjections combine with other words to form sentences, but not with finite verbs. Also, interjections like "so" and "what" are rarely used in the sentences.
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Noun
1. an abrupt emphatic exclamation expressing emotion
(synonym) ejaculation
(hypernym) exclamation, exclaiming
(derivation) interject, come in, interpose, put in, throw in, inject
2. the action of interjecting or interposing an action or remark that interrupts
(synonym) interposition, interpolation, interpellation
(hypernym) break, interruption, disruption, gap
(derivation) interject, come in, interpose, put in, throw in, inject

n. interjection, interjecting of a word or phrase in a conversation; word or phrase inserted in a conversation; exclamation expressing emotion or feeling (Grammar)

(n.)
The act of interjecting or throwing between; also, that which is interjected.
  

(n.)
A word or form of speech thrown in to express emotion or feeling, as O! Alas! Ha ha! Begone! etc. Compare Exclamation.