exactly when | just if

If and only if (i.e., necessary and sufficient). The terms "just if" or "exactly when" are sometimes used instead. A iff B is written symbolically as A<->B, A⇔B, A equilibrium B, or A ⟺ B. A iff B is also equivalent to A⇒B together with B⇒A, where the symbol ⇒ denotes "implies." Other equivalent terms are "A is equivalent to B" (A congruent B) and "A XNOR B." J. H. Conway believes that the word originated with P. Halmos and was transmitted through Kelley, where it is stated "F is equicontinuous at x iff there is a neighborhood of x whose image under every image of F is small."

at least one | equivalent | exactly one | implies | necessary | sufficient | XNOR