One of the Eilenberg-Steenrod axioms which states that, if
is a space with subspaces and such that the set closure of
is contained in the interior of , then the inclusion
induces an isomorphism .
More things to try:
Weisstein, Eric W. "Excision Axiom." From
MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/ExcisionAxiom.html