1) Condition 2) Must 3) Necessary 4) Need 5) Obligation 6) Obligatory 7) Proviso 8) Requirement 9) Sine qua non 10) Stipulation 1) Needed thing 2) Obligatory 3) Required beforehand 
• (n.) Something previously required, or necessary to an end or effect proposed. • (a.) Previously required; necessary as a preliminary to any proposed effect or end; as, prerequisite conditions of success.
A course which must be completed before a student is allowed to register for a more advanced course.
Course required as preparation for entry into a more advanced academic course or program.
Pre·req'ui·site adjective Previously required; necessary as a preliminary to any proposed effect or end; as, prerequisite conditions of success. 
[adj] - required as a prior condition or course of study 2. [n] - something that is required in advance
prerequisite 1. Required beforehand, especially as a necessary condition for something that follows. 2. An object, quality, or condition that is required in order for something else to happen. Related before-word units: ante-; antero-; anti-; pro-. Related 'time' units: aevum, evu...
adjective required as a prior condition or course of study
required beforehand: a prerequisite fund of knowledge.
something that is needed or obligatory in advance[SAT terms] something that is required in advance something that is required in advance