The Chicago Manual of Style:
When a word or term is not used functionally but is referred to as the word or term itself, it is either italicized or enclosed in quotation marks. Proper nouns used as words, on the other hand, are usually set in roman.
- The term critical mass is more often used metaphorically than literally.
- What is meant by neurobotics?
- The i in the name iPod is supposed to invoke the Internet.
Although italics are the traditional choice, quotation marks may be more appropriate in certain contexts. (And in some electronic environments, quotation marks may be more portable or otherwise practical than italics.) In the first example below, italics set off the foreign term, and quotation marks are used for the English. In the second example, quotation marks help to convey the idea of speech.
- The Spanish verbs ser and estar are both rendered by "to be."
- Many people say "I" even when "me" would be more correct.
Even though this topic is a matter of style/convention (it seems that even The CMS is reluctant to give a definite opinion), is there a source for a comprehensive discussion as to why one should prefer one to another?