We present and analyze spectra of the Type IIn supernova 1994W obtained between 18 and 202 days after explosion. During the first 100 days the line profiles are composed of three major components: (i) narrow P Cygni lines with absorption minima at −700 km s−1; (ii) broad emission lines with blue velocity at zero intensity ~ 4000 km s−1; (iii) broad, smooth, extended wings most apparent in Hα. These components are identified with the expanding circumstellar (CS) envelope , shocked cool gas in the forward postshock region, and multiple Thomson scattering in the CS envelope, respectively. The absence of broad P Cygni lines from the supernova (SN) is the result of the formation of an optically thick, cool, dense shell at the interface of the ejecta and the CS envelope. Models of the SN deceleration and Thomson scattering wings are used to recover the Thomson optical depth of the CS envelope, τT ≥ 2.5 during first month, its density (n ~ 109 cm-3) and radial extent, ~ (4 — 5) × 1015 cm. The plateau-like SN light curve, which we reproduce by a hydrodynamical model, is powered by a combination of internal energy leakage after the explosion of an extended presupernova (~ 1015 cm) and subsequent luminosity from circumstellar interaction. We recover the pre-explosion kinematics of the CS envelope and find it to be close to homologous expansion with outmost velocity ≈ 1100 km s-1 and a kinematic age of ~ 1.5 yr. The high mass (≈ 0.4 M ⊙) and kinetic energy (≈ 2 × 1048 erg) of the CS envelope combined with small age strongly suggest that the CS envelope was explosively ejected only a few years before the SN explosion.