The title of a section in my paper is
Bipolar Disorder Def.
Should it be "Def." or "def."?
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What is "def" an abbreviation of? Defined, definition, definite or definitely?
Is there a good reason why an abbreviation must be used in a title?
It is my belief that abbreviations should be avoided in titles unless they are works of literature or newspaper articles. Write the term in full, it's very likely that you're only going to use it once.