  In this work we consider how a parton distribution function, with an explicit
transverse momentum dependence can be properly defined in a
regularization-scheme independent manner. We argue that by considering a
factorized form of the transverse momentum dependent spectrum for the
production of a heavy lepton pair in Drell-Yan reaction, one should first split
the relevant soft function into two boost invariant contributions. When those
soft contributions are added to the pure collinear contributions, well-defined
hadronic matrix elements emerge, i.e., the transverse momentum dependent
distributions. We also perform a comparison with Collins' definition.
