It refers to the shortened form of a word or phrase.
However, abbreviations themselves can function as different parts of speech depending on their use in a sentence:
* Noun: "The FBI is investigating the case." (FBI is an abbreviation used as the subject of the sentence)
* Adjective: "He has a PhD in physics." (PhD is an abbreviation modifying the noun "degree")
* Verb: "RSVP by Friday." (RSVP is an abbreviation acting as the verb in the sentence)
So, while the word "abbreviation" is a noun, the abbreviations themselves can take on various roles within a sentence.