In quantum field theory, a contact term is a radiatively induced point-like interaction. These typically occur when the vertex for the emission of a massless particle such as a photon, a graviton, or a gluon, is proportional to q 2 (the invariant momentum of the radiated particle). This factor cancels the 1 / q 2 of the Feynman propagator, and causes the exchange of the massless particle to produce a point-like δ δ -function effective interaction, rather than the usual ∼ ∼ 1 / r long-range potential. A notable example occurs in the weak interactions where a W-boson radiative correction to a gluon vertex produces a q 2 term, leading to what is known as a "penguin" interaction. The contact term then generates a correction to the full action of the theory.