The broadening of a lineshape as a result of collisions, a spread of temperatures or velocities, etc. The "natural lineshape" is a Lorentzian lineshape, because an electron may radiate over a finite amount of time. Collisions then increase the width in a Lorentzian lineshape. The Doppler lineshape results from collisions, and is Gaussian in distribution. The convolution of Lorentzian and Doppler lineshapes is called a Voigt lineshape.
