The definition is even "a couple and their dependent children, regarded as a basic social unit." AKA family, So why "Nuclear?"
Also I'm posting to this sub because when I think of "Nuclear Family" I think of those old 40-50's drawings (Which is not what comes up when you google it)
The nucleus is the center of the atom. The nuclear family is the the central family with farther relations being, well, farther. So if the family was mapped out as an atom...you get it.