If I am defining in my paper specialized terminology that I learned in class, do I need to cite a source?

Note: This post relates to content in the eighth edition of the MLA Handbook. For up-to-date guidance, see the ninth edition of the MLA Handbook.

No. Specialized terminology learned in a class can be considered a kind of common knowledge in the context of an essay written for that class, so a works-cited-list entry is not needed. You can read more from the Style Center about what kinds of information do not require entries in works-cited lists.