I use "open source" personally, but it depends if there is ambiguity. "Open-source" is a compound modifier which are used to avoid ambiguity. Example: "Man-ea

I use "open source" personally, but it depends if there is ambiguity. "Open-source" is a compound modifier which are used to avoid ambiguity. Example: "Man-eating shark" could be read "man eating shark" without the hyphen, which can change the meaning to a man eating a shark instead of a shark eating a man. Wikipedia switches constantly: http://en.wikipedia.org/wiki/Open_source http://en.wikipedia.org/wiki/Open_source "open-source" occurs 138 times "open source" occurs 119 times I can't think of a scenario where "open source" leads to ambiguity. I would argue "open-source" removes any chance of ambiguity and is the better choice, but for aesthetic purposes I prefer "open source".