Anisotropy is a term for direction dependence of a property as opposed to Isotropy which means a property being identical in all directions.
In Cosmology, the term is used to describe the uneven Frequency (interpreted as uneven black-body Temperature) of the Cosmic Microwave Background. Wilkinson Microwave Anisotropy Probe is designed to map this. The uneven temperature is hypothesized to result from density variations in the universe from the era when the CMB was produced, and the angular scale and degree of the variations are studied to learn about the early universe.
Angular Power Spectrum
Cosmic Microwave Background (CMB)
Solar and Heliospheric Observatory (SOHO)
Sachs-Wolfe Effect (SWE)
Wilkinson Microwave Anisotropy Probe (WMAP)