  The concepts of pole mass and width are extended to unstable fermions in the
general framework of parity-nonconserving gauge theories, such as the Standard
Model. In contrast with the conventional on-shell definitions, these concepts
are gauge independent and avoid severe unphysical singularities, properties of
great importance since most fundamental fermions in nature are unstable
particles. General expressions for the unrenormalized and renormalized dressed
propagators of unstable fermions and their field-renormalization constants are
presented.
