o·blig·a·to·ry (ə-blĭgə-tôr′ē, ŏblĭ-gə-)
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.
The American Heritage® Dictionary of the English Language, Fifth Edition copyright ©2019 by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Indo-European & Semitic Roots Appendices
Thousands of entries in the dictionary include etymologies that trace their origins back to reconstructed proto-languages. You can obtain more information about these forms in our online appendices:
The Indo-European appendix covers nearly half of the Indo-European roots that have left their mark on English words. A more complete treatment of Indo-European roots and the English words derived from them is available in our Dictionary of Indo-European Roots.