Kinetic term

In quantum field theory, a kinetic term is any term in the Lagrangian that is bilinear in the fields and has at least one derivative. Fields with kinetic terms are dynamical and together with mass terms define a free field theory. Their form is primarily determined by the spin of the fields along with other constraints such as unitarity and Lorentz invariance. Non-standard kinetic terms that break unitarity or are not positive-definite occur, such as when formulating ghost fields, in some models of cosmology, in condensed matter systems, and for non-unitary conformal field theories.