What does this colon do in an enum declaration?

I did a search for this question thinking that somebody must have asked it before. I did not turn up any results, so if it has been, please post the link and feel free to close the question.

I ran across this code in EASTL:

enum : size_type {   // size_type = size_t                   
                npos     = (size_type)-1,
                kMaxSize = (size_type)-2
            };

I have never encountered an enum declaration like that. What does the : do in this case?