I don't really like the term "white space" either, because the background is not necessarily white. But, that is the word we use. At least, making it into a com

I don't really like the term "white space" either, because the background is not necessarily white. But, that is the word we use. At least, making it into a compound like "whitespace" might be better, in case in some contexts you need to avoid to confuse with "white space" in pictures where much of it is left white, and also "whitespace" might be better for dictionary stuff. A variant of PNG called PNG-16 uses the term "black magic", and it is appropriate there, because it is indeed black!