pre·req·ui·site /(ˌ)priˈrɛkwəzət/
  (a.)預先非有不可的,必須的首要事物,必要條件
  prerequisite
  先決條件(必備條件)
  Pre·req·ui·site a. Previously required; necessary as a preliminary to any proposed effect or end; as, prerequisite conditions of success.
  Pre·req·ui·site, n. Something previously required, or necessary to an end or effect proposed.
     The necessary prerequisites of freedom.   -- Goldsmith.
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  prerequisite
       adj : required as a prior condition or course of study
       n : something that is required in advance; "Latin was a
           prerequisite for admission" [syn: requirement]