2. n. 
The flow of reservoir fluids from one zone to another. Crossflow can occur when a lost returns event is followed by a well control event. The higher pressured reservoir fluid flows out of the formation, travels along the wellbore to a lower pressured formation, and then flows into the lower pressure formation.
3. n. 
A condition that exists when two production zones with dissimilar pressure characteristics are allowed to communicate during production. Reservoir fluid from the high-pressure zone will flow preferentially to the low-pressure zone rather than up the production conduit unless the production parameters are closely controlled.