Dipole Anisotropy
Dipole anisotropy is a term that means a deviation
from spherical symmetry that can be described simply by something being larger in
one direction and smaller in the exact opposite direction. This is the type of
deviation from smoothness expected for Doppler shifts due to uniform motion of the Earth
with respect to the CMB.
More complex anisotropies cannot simply be described by being larger in one direction and
smaller in the opposite direction. We may search for those by first subtracting
the dipole component due to Earth's motion and then seeing if any lack of uniformity remains.