I'm posting this as a question/discussion rather than to the synonyms thread, as I'm not entirely sure on the exact American spelling. Perhaps a hyphen would also be better (ie. "gray water") rather than one combined word.
It should be a combined word, not hyphenated or with spaces.
greywater used in many American contexts. (Not sure if it is used more often than
There's no real consistency -- it's spelled both ways on EPA's site, for example. (They also spell it with spaces sometimes.)
Bottom line: it's a gray area, I don't think it matters much...
(Sorry, couldn't resist.)
For this kind of thing (also lawnmower vs. lawn-mower) I do a quoted Google search and declare the one with more hits the winner. I'm sure there are exceptions where this leads astray (a certain Republican presidential candidate comes to mind), but it's usually a good indicator.