It depends on mutual agreements. In the case of standard notation for
SI units, for example, it was an international committee. Not
everything is defined formally. I do not think that it was necessary
to define a standard resistance. (The R, L, C, model, described by
John D. was sufficient at high frequencies. In that context R was
indeed considered not to depend on f. I never worked with frequencies
at which the skin effect was significant.)
Ludwik Kowalski, a retired physics teacher
5 Horizon Road, Apt. 2702, Fort Lee, NJ, 07024, USA
Also an amateur journalist at http://csam.montclair.edu/~kowalski/cf/