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 (the invariant momentum of the radiated particle).This factor cancels the of the Feynman propagator, and causes the exchange of the massless particle to produce a point-like -function effective interaction, rather than the usual long-range potential. A notable example occurs in the weak interactions where a W-boson radiative correction to a gluon vertex produces a term, leading towhat is known as a "penguin" interaction. The contact term then generates a correction to the full action of the theory.