We define the collinear factorization scheme, which absorbs only the
collinear physics into the parton distribution functions.  In order to
isolate the collinear physics, we introduce a procedure to combine
real and virtual corrections, canceling infrared singularities prior
to integration.  In the collinear scheme, the factorization scale \mu
has a simple physical interpretation as a collinear cutoff. We present
a method for choosing the factorization scale and apply it to the
Drell-Yan process; we find \mu \approx Q/2, where Q is the
vector-boson invariant mass.  We show that, for a wide variety of
collision energies and Q, the radiative corrections are small in the
collinear scheme for this choice of factorization scale.