Merriam-Webster Collegiate Dictionary - necessity

 noun  (plural -ties)  Etymology: Middle English necessite, from Anglo-French necessite, from Latin necessitat-, necessitas, from necesse  Date: 14th century  1. the quality or state of being necessary  2.  a. pressure of circumstance  b. physical or moral compulsion  c. impossibility of a contrary order or condition  3. the quality or state of being in need; especially poverty  4.  a. something that is necessary ; requirement  b. an urgent need or desire