  We present and analyse spectra of the Type IIn supernova 1994W obtained
between 18 and 203 days after explosion. During the luminous phase (first 100
d) the line profiles are composed of three major components: (i) narrow P-Cygni
lines with the absorption minima at -700 km/s; (ii) broad emission lines with
BVZI ~4000 km/s; and (iii) broad, smooth wings, most apparent in H-alpha. These
components are identified with an expanding circumstellar (CS) envelope,
shocked cool gas in the forward post-shock region, and multiple Thomson
scattering in the CS envelope, respectively. The absence of broad P-Cygni lines
from the supernova is the result of the formation of an optically thick, cool,
dense shell at the interface of the ejecta and the CS envelope. We model the
supernova deceleration and Thomson scattering wings to recover the density,
radial extent and Thomson optical depth of the CS envelope during the first
month. We reproduce the light curve with a hydrodynamical model and find it to
be powered by a combination of internal energy leakage after the explosion of
an extended pre-supernova (~10^15 cm) and luminosity from circumstellar
interaction. We recover the pre-explosion kinematics of the CS envelope: it is
close to homologous expansion with outer velocity ~1100 km/s and a kinematic
age of ~1.5 yr. The CS envelope's high mass and kinetic energy, combined with
its small age, strongly suggest that the CS envelope was explosively ejected
about 1.5 yr before the supernova explosion.
