1. The optical counterpart of an illuminated object formed by light rays as they converge after traversing an optical system. 2. Using a virtual image as a seismic source in tracing raypaths through a constant velocity medium, which is often easier than tracing from the actual source. The virtual image for a reflection is located as far below a plane reflector as the source is above it, ‘‘below’’ meaning at right angles to the reflector. Often called image point. See Figure I-2. 3. A common-image point (CIP), the point where prestack migrated data reinforce each other. 4. A method used in electrical modeling whereby a plane boundary is replaced by an image whose magnitude is k=(ρ2–ρ1)/(ρ2+ρ1), where ρ1,ρ2 are the resistivities on opposite sides of the boundary. The result in medium 1 is that the potential and current distribution in the zone of interest is unchanged. The objective is to facilitate computation of the potential distribution.