Two graphs G and H are called isomorphic, denoted G≅H, if there exists a bijection f:V(G)→V(H) such that any two vertices u,v∈V(G) are adjacent in G if and only if f(u),f(v) are adjacent in H.
Formally, f preserves the adjacency relation:
{u,v}∈E(G)⟺{f(u),f(v)}∈E(H)