When studying quantum electrodynamics (QED), the theory that describes the electromagnetic interaction, the Feynman rules provide a graphical representation of the mathematical calculations involved in particle interactions. In these rules, fermions are represented by lines with arrows indicating their direction of propagation.
An "internal fermion" refers to a fermion line that appears within a Feynman diagram but does not correspond to an external particle. In other words, it represents a virtual particle that is involved in intermediate steps of a particle interaction, rather than a particle that is observed directly in the final state.
Internal fermion lines can appear in various processes within QED, such as electron-electron scattering or electron-photon scattering. These lines typically connect with other fermion lines and/or gauge boson lines, representing the exchange of particles during the interaction. The internal fermions play a crucial role in calculating the probability amplitudes for these processes and contribute to the overall understanding of the quantum behavior of particles in the electromagnetic force.