If you were to make a vegetable beef soup and used beef stock as the primary flavor and liquid, the beef stock would be considered the "base" of the soup.
In chicken curry, the curry is the primary flavor, thus it's the base.
In using a roux as the thickening/flavor agent in making gravy, the roux is considered to be the base.
The predominant flavor upon which all of the other flavors are layered, is considered to be the "base".
I hope I made this clear enough, and apologize if I didn't.