cover
family of subsets of a set whose union equals the whole set
tessellation
tiling of a plane using one or more geometric shapes, called tiles, with no overlaps and no gaps
exact cover
disjoint family of sets, drawn from a larger collection, with the same union as the whole collection
open cover
cover of a topological space consisting solely of open subsets
Lakes of Wada
three disjoint sets that share a common boundary