prerequisite

Short definition from Wiktionary

Adjective

1. 1
   Required as a prior condition of something else; necessary or indispensable.

Noun

1. 1
   Something that is required as necessary or indispensable, or as a prior condition of something else.
2. 2
   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.