  The renormalization properties of unintegrated (transverse-momentum
dependent) parton distribution functions (TMD PDF's) are used for analyzing
their completely gauge-invariant definition. To this end, the UV anomalous
dimension is calculated at the one-loop order in the light-cone gauge and a
consistent treatment of the extra singularities, which produce undesirable
contributions in the anomalous dimensions, is given. The generalized definition
of a TMD PDF, based on the renormalization procedure for the Wilson
exponentials with obstructions, is proposed. The reduction of the re-defined
TMD PDF to the integrated PDF's, as well as their probabilistic interpretation,
are discussed.
