Hi, I have troubles understanding the difference between linear and non linear matter power spectrum. These words are commonly used in the litterature, but I have found no definitions yet. My understanding is that there is one definition of the power spectrum for matter distribution, basically as the Fourier transform of the 2-point correlation function of the density field. It covers both very large scales where linear approximation for the dynamics should be accurate, while at small scales the non linear gravitational collapse heavily impacts the dynamics. Still, we have only one power spectrum. So, what does linear or nonlinear power spectrum refers to? (in terms of maths, I mean) Thanks for your help!