51st state refers to a place or territory that is not one of the 50 states of the united states, but people think about making it the 51st state. it is sometimes a joke term in american politics, and sometimes a realistic one. it is usually said about the possibility of puerto rico or other u.s. territories becoming part of the united states. when used to refer to other countries, it is a joke, usually about canada. there have been attempts by people in the united states to get canada to join them (like in the articles of confederation), but canada has never accepted these offers. it is also sometimes used to refer to countries that seem to be controlled by the united states. before the year 1959 when alaska and hawaii became states, the sentence "the 49th state" was used.