A Void is an empty space between Large Scale Structures, such as a sheet, wall, or Galaxy Filament, with a diameter on the order of 10 or 100 megaparsecs. They were first pointed out in 1978 by Stephen Gregory and Laird A. Thompson.
Voids are of interest in Cosmology, which aims to describe the distribution of matter in the universe. Unexpectedly many or large voids indicate some early influence that caused non-homogeneity in matter's distribution.
Large voids (e.g., with diameters in the 100 megaparsec range) are called Supervoids. The largest known supervoid is the Great Void or Eridanus Supervoid, which has a diameter on the order of a billion light-years.