We call it "solfège".

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.

Akshually... According to Wikipédia FR, the French equivalent of Sight Reading is "Lecture à vue" [1].

Notes:

  - I'm French; 
  - I learnt to play the piano as a teenager, and I'm trying to get back to it again as at 45+; 
  - I too discovered the "sight reading" term online, but that was long ago (as I recall, through a video of an adult piano player doing "live sight-reading" versions of songs he didn't know beforehand, such as the Mario theme [2];

... and YET neither have I ever heard of nor used "Lecture à vue" (that I remember of). I too would use "apprendre le solfège" (EN: learn solfège) when saying that I'm learning to read the notes on sheet music while playing at the same time.

[1]: https://fr.wikipedia.org/wiki/Lecture_%C3%A0_vue (change language for confirmation)

[2]: Possibly this one? https://www.youtube.com/watch?v=b9IkpUYlOx8

Solmization, I think is maybe what you are looking for? It is where you sing a song using the Solfege (Do/Re/Mi....)?

Though I wouldn't say that it is the same as "sight reading" -- but maybe "sight singing"?