  We report an s-wave collisional frequency shift of an atomic clock based on
fermions. In contrast to bosons, the fermion clock shift is insensitive to the
population difference of the clock states, set by the first pulse area in
Ramsey spectroscopy, \theta_1. The fermion shift instead depends strongly on
the second pulse area \theta_2. It allows the shift to be canceled, nominally
at \theta_2 = \pi/2, but correlations shift the null to slightly larger
\theta_2. The shift applies to optical lattice clocks and increases with the
spatial inhomogeneity of the clock excitation field, naturally large at optical
frequencies.
