Well I'm probably wrong, but to me its synonymous with application security.
Application security actually seems a bit hard to define. Code may be secure and still be used to harm others. It may be secure but and still used in ways it wasn't assigned to allow.
I guess you need a set of constraints about what the code should and should not do in order to define what secure means for it?
(Code safety sounds a bit different to me, don't know if that's based on anything. I'd interpret it as not containing bugs, and perhaps even it not being to easy to introduce bugs. Largely synonymous with code quality I guess. And mostly a superset of security, as most safety concerns are bugs but not all bugs are safety concerns. Perhaps I'm inventing my own meaning for words though...).