  Determining the nature of electronic states in doped Mott insulators remains
a challenging task. In the case of tetragonal La$_{2-x}$Sr$_{x}$NiO$_{4}$, the
occurrence of diagonal charge and spin stripe order in the ground state is now
well established. In contrast, the nature of the high-temperature "disordered"
state from which the stripe order develops has long been a subject of
controversy, with considerable speculation regarding a polaronic liquid.
Following on the recent detection of dynamic charge stripes, we use neutron
scattering measurements on an $x=0.25$ crystal to demonstrate that the
dispersion of the charge stripe excitations is anisotropic. This observation
provides compelling evidence for the presence of electronic nematic order.
