In physics, a kinetic term is the part of the Lagrangian that is bilinear in the fields (and for nonlinear sigma models, they are not even bilinear^{[clarification needed]}), and usually contains two derivatives with respect to time (or space); in the case of fermions, the kinetic term usually has one derivative only. The equation of motion derived from such a Lagrangian contains differential operators which are generated by the kinetic term. Unitarity requires kinetic terms to be positive.

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In mechanics, the kinetic term is

${\displaystyle T={\frac {1}{2}}{\dot {x}}^{2}={\frac {1}{2}}\left({\frac {\partial x}{\partial t}}\right)^{2}.}$

In quantum field theory, the kinetic terms for real scalar fields, electromagnetic field and Dirac field are

${\displaystyle T={\frac {1}{2}}\partial _{\mu }\Phi \partial ^{\mu }\Phi +{\frac {1}{4g^{2}}}F_{\mu \nu }F^{\mu \nu }+i{\bar {\psi }}\gamma ^{\mu }\partial _{\mu }\psi .}$

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