A hole in the superstructure of the universe approximately 45 megaparsecs in diameter. Essentially a great big empty spot in the universe containing 2.7x10^17 cubic lightyears of space and only 53 known galaxies.
First discovered by Kirshner in 1987 when a cosmological redshift survey showed a surprisingly large gap in the direction of the constellation Bootes. Later slice-style surveys showed that the void was much smaller than originally though, but still the largest one within a billion lightyears.
At the time in which is was discovered, it really irritated cosmologists who were still looking at a completely uniform cosmic background radiation and expecting the universe to have consistency pretty similar to pudding: approximately uniform in every direction. However, ultra-large scale mapping projects like Kirschner's, and later slice projects (like the CfA) showed that the universe looked a lot more like soap-suds, with most galaxies concentrated in strands or sheets (like The Great Wall and the Stickman) surrounding great voids of virtual emptyness. The exact mechanism behind the formation of these structures is still unclear, but the general consensus is that the slight quantum fluctuations that we now know to have existed shortly after the Big Bang probably had some hand in it.