(dā’ d∂m) 1. An arbitrary reference surface, the reduction to which minimizes local topographic and near-surface effects. 2. Elevation datum, the reference level for elevation measurements, often sea level. 3. Seismic datum, an arbitrary reference surface, the reduction to which minimizes local topographic and near-surface effects. Seismic times and velocity determinations are referred to the datum plane (usually but not necessarily horizontal and planar) as if sources and geophones had been located on the datum plane and as if no low-velocity layer existed. 4. A paleo-datum is used in an attempt to restore strata or reflections to the structural positions held at some ancient time.