virtually

1. (algebra) Of a subgroup of finite index.Examples: "In this case W has a diagram with 2 vertices and one edge labelled ∞, and is therefore the infinite dihedral group. Thus W is virtually ℤ."; "In this paper, we are mainly concerned with homogeneity in the class of finitely generated virtually free groups, i.e. finitely generated groups with a free subgroup of finite index."

2. (topology) Of a covering space of finite degree.Examples: "Another class of small manifolds consists of those that are not virtually Haken (a manifold is virtually Haken if it is finitely covered by a Haken manifold)."; "We call a hyperplane Y in a VH-complex X virtually clean if there exists a finite-sheeted covering map p : X̂ → X and a connected component Y ⊆ p⁻¹Y such that the hyperplane Ŷ of X̂ is clean."