1. The difference between a predicted value and a measured value. 2. The difference of values at identical points on intersecting seismic lines, or of values determined by independent methods. The difference from zero of the algebraic sum of measured differences around a loop. 3. The difference obtained on carrying a reflection, phantom, or some other measured quantity around a loop. 4. The difference between the depths predicted from seismic data and measured in a borehole.