My question is, should synonyms be italicized or enclosed in quotation marks? Are the synonyms considered words or concepts?
Hello, Julie Link, and welcome to the Grammar Exchange.
I guess by "concepts" you mean to say "words used to mean what they mean," rather than referring to them as words. Here is an interesting discussion on the matter:
Words as words: As already noted, words used as words are usually italicized. This helps forestall confusion when these words are not used in the usual manner.
Examples: The word haberdashery has gone out of style.
Edith wasn’t sure what lugubrious meant, but it sounded slimy to her.
Notice that above I wrote "concepts" (and now I'm using it again) as a word, and used quotes. I think italics could also have been used, but the quotes work, perhaps because I was quoting the word you used. I agree that definitions should be enclosed between quotes, as I did above: "words used to mean what they mean."
I think the key to decide whether synonyms are being used as words or as concepts lies in the verb by means of which they are introduced:
Synonyms for uphold include lift, take up, and upraise. (Here lift, take up, and upraise are being used as words.)
Semantically speaking, uphold is close / equivalent to lift, take up, and upraise. (Same thing.)
The word uphold means / can be defined as "to lift, to take up or to upraise." (Here the words are being used as concepts.)