What's the interconnection between UCD & UX (User-Centered Design & User Experience Design)?

Having researched all I could find about UCD, I’m a little bit confused as to how it relates to User Experience Design (UX).

Wikipedia seems to suggest that if there is a question about development of one single (1) interaction, it’s a matter of User-Centered Design, and when it becomes more than one interaction—i.e. a system of any kind—we’re dealing with User Experience Design.

But, there can’t really be the sole definition of interaction quantity that separates the methodologies apart, or?

If anyone can please give me a push in the right direction here, I’d be very thankful.