We present an analysis of UV and optical spectra of NGC 4151 obtained at high spectral and angular resolutions with the Hubble Space Telescope Space Telescope Imaging Spectrograph (HST STIS). We identify a kinematic component of the emission lines that has a width of 1170 km s^-1 (FWHM), intermediate between those from the broad and narrow (emission) line regions (BLRs and NLRs). We present evidence that these emission lines arise from the same gas responsible for most of the high-column UV and X-ray absorption (component "D+E") that we see in outflow at a distance of ~0.1 pc from the central nucleus. The gas in this intermediate-line region (ILR) shields the NLR and has a global covering factor of ~0.4, based on the observed C IV fluxes, indicating mass outflow over a large solid angle centered on the accretion disk's axis. A large transverse velocity (v_T 2100 km s^-1) compared to the radial velocity centroid (v_r = -490 km s^-1) indicates that the kinematics is dominated by rotation at this distance, but has a significant outflow component. The mass outflow rate at 0.1 pc is ~0.16 M yr^-1, which is about 10 times the accretion rate. Based on physical conditions in the gas and dynamical considerations, models that invoke magnetocentrifugal acceleration (e.g., in an accretion disk wind) are favored over those that rely on radiation driving or thermal expansion as the principal driving mechanism for the mass outflow.