A nest is a dwelling in which multiple vampires live together. On the HBO original series True Blood, there are several nests that appear, or are mentioned, throughout the series' first, second, and fifth seasons.
Nests are described (by Bill in the first season) as causing the vampires living in them to become more cruel. Pam implies this is linked to their drinking each other's blood and assuming the personality traits of the strongest, and usually cruelest, member. ("Sunset")