Huh. My instinct was that (in English), "solfège" is just the name of the collection of notes. (The same way that "alphabet" is the name of the collection of letters beginning "A B C D E ...".)

And interestingly enough, the English wikipedia page completely backs me up on that. https://en.wikipedia.org/wiki/Solf%C3%A8ge ("Solfège is a form of solmization, though the two terms are sometimes used interchangeably.")

However, the English wiktionary page gives no gloss other than "a method of sight singing".

Merriam-Webster strongly supports the wikipedia definition, but it does note that "a method of sight-reading music" is an alternative, non-primary meaning.