Ms (British English only) or Ms. (American English and British English) (normally /ˈmɪz/ when stressed; when unstressed, either /ˈmɪz/, /mɨz/, /məz/ or /məs/) is an English honorific used with the last name or full name of a woman. Ms. is intended as a default form of address for women, regardless of marital status. Like Miss and Mrs., the term Ms. is ultimately, though indirectly, derived from the female English title for all women, Mistress. It was invented (and independently reinvented) in the 20th century. Either Mss. or Mses. may be used as the plural. Alternatively, if using the traditional French plurals ("Messieurs" for Mr., and "Mesdemoiselles" for Miss) one may use "Mesdames" (abbreviation "Mmes."), which is also the plural for "Mrs."

Read more about Ms.:  Historical Development and Revival of The Term, Usage