Reverse circulation occurs when fluid is pumped down an annular space and returns to the surface through the tubular forming the inner wall of the annular space. This is opposite of normal circulation wherein fluid is pumped down the inner tubular pipe and returns up the annular space.
Source: API RP 54, Recommended Practice for Occupational Safety for Oil and Gas Well Drilling and Servicing Operations, Third Edition, August 1999 (2007). Global Standards
Circulation of drilling fluid down the annulus and up through the drill string.
Source: IADC UBO / MPD Glossary, December 2011. Global Standards