The amount by which formation pressure exceeds pressure exerted by the hydrostatic head of fluid in the well bore.
Source: API RP 59, Recommended Practice for Well Control Operations, Second Edition, May 2006. Global Standards
A condition where the pressure exerted in the wellbore is less than the pore pressure in any part of the exposed formations.
Source: IADC UBO / MPD Glossary, December 2011. Global Standards