The counterterm in quantum field theory is a term added to the seed Lagrangian for the subsequent elimination of ultraviolet divergences in calculating the higher orders of the perturbation theory . The explicit form of the counterterms depends on the particular regularization and subtraction scheme.
In the renormalizable theory, counterterms have, as a rule, the same form as the terms of the original Lagrangian. In the case of non-renormalizable theories, in order to eliminate divergences in ever higher orders of perturbation theory, it is necessary to introduce more and more counterterms, which does not allow one to obtain a closed expression for the Lagrangian.