Definition of Underbalance Drilling
"When the hydrostatic head of a drilling fluid is intentionally designed to be lower than the pressure of the formation being drilled, the operation will be considered underbalanced drilling. The hydrostatic head may be naturally less than the formation pressure or it can be induced. Whether natural or induced, this may result in an influx of formation fluids which must be circulated from the wellbore and controlled at surface."
- AEUB document ID94-3