A contraction of is a shortened form of a word or phrase. ISNT is a contraction of 'is not' and is commonly used in informal writing. TIS is a contraction of 'it is' and is also used in informal writing. SPASM is a contraction of 'spontaneous muscular contraction' and is used to describe an involuntary muscle contraction. SHORTFOR is a contraction of 'short for' and is used to indicate that a word or phrase is an abbreviation of a longer phrase.