A resonance that can occur when two systems described by quantum mechanics are able to interact. For example, a bound state can form if the binding energy holding two atoms together is equal to the sum of the kinetic energies of the two atoms when they collide. In some systems it is possible to alter the parameters to produce a Feshbach resonance. Although first suggested by Herman Feshbach (1917–2000) in the context of nuclear theory, the concept of a Feshbach resonance also occurs with ultracold atoms.