A person with whom one shared a special relationship that is somewhat more than simply as friend, but not not quite the level of "significant other." May be former or future significant other, or even someone whom will never have that status. Some "best friends" qualify, but not nearly all do. A sexual component to the relationship is quite common, though it is not required.
Note: A sexual component does not automatically raise a friend to moderately significant other status, rather it is the strength of the bond between the two parties.
"I haven't had a girlfriend in a while, so I've been spending a lot of time with my moderately significant other, and we've even gone to a few weddings together. But somehow, we've never quite gone out."