In particle physics, the Breit frame (also known as infinite-momentum frame or IMF) is a frame of reference used to describe scattering experiments of the form ⁠${\displaystyle \textstyle A+B\rightarrow A+\sum C_{i}}$⁠, that is experiments in which particle ⁠${\displaystyle A}$⁠ scatters off particle ⁠${\displaystyle B}$⁠, possibly producing particles ${\displaystyle C_{i}}$ in the process. The frame is defined so that the particle A has its momentum reversed in the scattering process.

Another way of understanding the Breit frame is to look at the elastic scattering ⁠${\displaystyle A+\gamma \rightarrow A'}$⁠. The Breit frame is defined as the frame in which ⁠${\displaystyle {\vec {p}}_{A}+{\vec {p}}_{A'}=0}$⁠. There are different occasions when Breit frame can be useful, e.g., in measuring the electromagnetic form factor of a hadron, ⁠${\displaystyle A}$⁠ is the scattered hadron; while for deep inelastic scattering process, the elastically scattered parton should be considered as ⁠${\displaystyle A}$⁠. It is only in the latter case the Breit frame gets related to infinite-momentum frame.

It is named after the American physicist Gregory Breit.