Q: Generalizations of the field concept to triplets of elements In the 'Field' article at MathWorld there is a phrase that I don't understand:

It has been proven by Hilbert and Weierstrass that all generalizations of the field concept to triplets of elements are equivalent to the field of complex numbers.

The same sentence occurs here, at the description of the 'field-theory' tag.
What does this exactly mean? What is the field concept applied to triplets of elements? Which theorem this phrase refers to?
