An abbreviation is a shortened form of a word or phrase, by any method. It may consist of a group of letters or words taken from the full version of the word or phrase; for example, the word abbreviation can itself be represented by the abbreviation abbr., abbrv., or abbrev.; NPO, for nil per (by) os (mouth) is an abbreviated medical instruction. It may also consist of initials only, a mixture of initials and words, or words or letters representing words in another language. Some types of abbreviations are acronyms or grammatical contractions or crasis.