A manual for writers of research papers, theses, and dissertations, 7th edition - Kate L. Turabian 2007

Words defined as terms
Names, special terms, and titles of works
Part III. Style 20 spelling

To emphasize key terms that you define, italicize them on their first use; thereafter use roman type. You can use quotation marks (called “scare quotes”) to alert readers that you are using a term in a nonstandard or ironic way. When overused, both techniques become less effective.

The two chief tactics of this group, obstructionism and misinformation, require careful analysis.

Government “efficiency” resulted in a huge deficit.

Italicize a term when you refer to it as a term.

The term critical mass is more often used metaphorically than literally.

How did she define the word existential?

but

A critical mass of students took existential philosophy.

Italicize letters referred to as letters, and present them in lowercase. Letters used to denote grades and to identify exemplars should be roman and capitalized. For plural forms of letters used in these ways, see 20.1.2.

Many of the place-names there begin with the letters h and k.

In her senior year, she received an A and six B's.

Imagine a group of interconnected persons: A knows B, B knows C, and C knows D.