Anabbreviationis a shortened form of a word or phrase. It can be a word in a reduced form, or the first letter or letters of a word or phrase.
Abbreviations like op. cit. and cf. are common in academic writing.
When the abbreviation is made up of first letters and is pronounced as a word, it is called anacronym.
NATO is the acronym for North Atlantic Treaty Organization.
When the abbreviation is pronounced as separate letters it can be called aninitialism.
BBC is an initialism for British Broadcasting Corporation.