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The term collisional broadening refers to a type of line broadening (mechanism creating width to spectral lines so they are not infinitely narrow) due to the effects of nearby particles which are charged or contain charged sub-particles (the close passes between moving particles that lead to some interaction are termed collisions). The term pressure broadening is also used; such broadening increases with the gas/plasma's pressure which reflects its density and particle momentum, such collisions occurring more frequently. A number of effects result from these interactions, due to various forces and mechanisms, which contribute to the spectral line shape in different ways, producing a combination (convolution) of a Lorentzian profile and a Gaussian profile. An example of such effects is a slight change in the energy-difference between an atom's electron orbitals due to the additional electric force from an electron that is near (either a free electron or one orbiting a nearby atom): the wavelength of a photon emitted or absorbed is thus slightly different.