Realistic NN interactions and many-body approaches have been used to calculate ground-state properties of nuclei with A=3, 4, 12, 16, 40, with particular emphasis on various kinds of momentum distributions. It is shown that at proper values of the relative (rel) and center-of-mass (c.m.) momenta, the two-nucleon momentum distribution n_A^{N_1N_2} (k_{rel}, K_{c.m.}, theta) exhibits the property of factorization, namely n_A^{N_1N_2} (k_{rel}, K_{c.m.}, theta) simeq n_{rel}(k_{rel}) n_(c.m.)( K{c.m.}). The factorization of the momentum distributions , bearing a universal character, results from a general property of realistic nuclear wave functions, namely their factorization at short inter-nucleon separations. The factorization of the two-nucleon momentum distribution allows one to develop the correlated part of the nucleon spectral function P(k,E) in terms of a convolution integral involving the product of the many-body, parameter-free relative and c.m. momentum distributions of a given nucleus. It is shown that: (i) the obtained spectral function perfectly satisfies the momentum sum rule, i.e. when it is integrated over the removal energy E, it fully reproduces the momentum distributions obtained from realistic many-body wave functions , (ii) in order to saturate the momentum sum rule at high values of the momentum (k simeq 5 fm^{-1}) the spectral function has to be integrate up to E simeq 400 MeV. To sum up a realistic, parameter-free many-body Spectral function has been developed such that : i) a phenomenological convolution spectral function developed in the past is fully justified from a many-body point of view , and (ii) the model dependence which might be present in calculations of inclusive electroweak processes could be reduced by the use of the convolution spectral function developed here.
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