o·blig·a·to·ry 
(ə-blĭgə-tôr′ē, ŏblĭ-gə-)
(ə-blĭgə-tôr′ē, ŏblĭ-gə-)adj.
1. Of the nature of an obligation; compulsory: Attendance is obligatory. Mathematics is an obligatory course.
2. Required as part of a routine course of action or cultural expectation; predictable: "wearing clothing like basketball shoes, baggy pants, and the obligatory baseball cap" (Tyler Stovall).
3. Biology Obligate.
o·blig′a·tori·ly adv.