  In this paper, we introduce a notion of the center and radius of a subset A
of metric space X. In the Euclidean spaces, this notion can be seen as the
extension of the center and radius of open/closed balls. The center and radius
of a finite product of subsets of metric spaces, and a finite union of subsets
of a metric space are also determined. For any subset A of metric space X,
there is a natural question to identify the open balls of X with the largest
radius that are entirely contained in A. To answer this question, we introduce
a notion of quasi-center and quasi-radius of a subset A of metric space X. We
prove that the center of the largest open balls contained in A belongs to the
quasi-center of A, and its radius is equal to the quasi-radius of A. In
particular, for the Euclidean spaces, we see that the center of largest open
balls contained in A belongs to the center of A, and its radius is equal to the
radius of A.
