Let f:(X,dX)→(Y,dY) be a function between metric spaces, and let A⊆X. We say f is continuous on A if the restriction f∣A:A→Y, with A carrying the subspace metric, is continuous. Equivalently, f∣A is continuous at every point a∈A.
Spelled out: for every a∈A and every ε>0, there exists δ>0 such that for all x∈A,
dX(x,a)<δ⇒dY(f(x),f(a))<ε.