  Optical and ultraviolet observations of the Type IIn supernova 1995N at
epochs between 321 and 1799 days after the explosion show three distinct
velocity components. The narrow lines come from circumstellar gas and show both
low and high ionization. This component has a low filling factor, and is
photoionized by X-rays from the shock. The intermediate component, which is
dominated by newly processed oxygen, originates in a shell with velocity of
2500-5000 km/s, and most likely comes from the ejecta. The hydrogen- and
helium-dominated gas has a low ionization, a high density, and velocities that
extend out to ~10,000 km/s. Strong signatures of Ly-alpha-pumped fluorescence
lines of Fe II are seen in the near-infrared and ultraviolet. The He/H ratio, ~
0.3 by number, and the nitrogen overabundance provide strong evidence for CNO
burning products. The fluxes of the broad hydrogen and helium lines decrease
considerably faster than the oxygen lines. The H-alpha line profile shows
strong evolution, with the red wing decreasing faster than the blue. Possible
scenarios, involving either a clumpy circumstellar medium, or an aspherical
distribution of the surrounding gas, are discussed based on the line profiles
and physical conditions. Finally, we propose that Type IIn supernovae have
their origin in red supergiants in a superwind phase.
