anything indispensable, A necessity or prerequisite; something required or obligatory, That which requires the doing of something; an authoritative or imperative command; an essential condition; claim

required as a prior condition or course of study, Previously required; necessary as a preliminary to any proposed effect or end, Required as a prior condition of something else; necessary or indispensable, noun_ Something that must be gained in order to gain something else, In education, a course or topic that must be completed before another course or topic can be started. May be colloquially referred to as a prereq, something that is required in advance