  Developments of main notions and concepts behind the MSW effect (1978 - 1985)
are described. They include (i) neutrino refraction, matter potential, and
evolution equation in matter, (ii) mixing in matter, resonance and level
crossing; (iii) adiabaticity condition and adiabatic propagation in matter with
varying density. They are in the basis of the resonance enhancement of
oscillations in matter with constant (nearly constant) density, and the
adiabatic flavor conversion in matter with slowly changing density. The former
is realized in matter of the Earth and can be used to establish neutrino mass
hierarchy. The latter provides solution to the solar neutrino problem and play
the key role in transformations of the supernova neutrinos.
