1. Onset of a transient wave, also called break (q.v.). 2. Sudden pressure change (usually expulsion of drilling fluid from the borehole). Can be caused by (a) lowering of fluid in the borehole, (b) too-rapid pulling of drillstem or large-diameter tools, (c) loss of circulation in vuggy, fractured formations, or (d) insufficient mud weight because of entering an overpressured zone, decrease in density, or dilution of drilling fluid. 3. Sudden change in drilling-fluid pressure caused by gas entering the drilling-fluid from a porous formation.