A deviation is a departure from an approved protocol or approved procedures after the execution of an action. In other words it is a failure to follow specified/approved procedures or protocol instructions. A deviation is an unapproved change.
A test case discrepancy is a discrepancy between a computed, observed or measured value or condition and the true, specified or theoretically correct value or condition. Anything observed in the documentation or operation of the software that deviates from expectations based on previously verified software products or reference documents.
In short, a discrepancy is when the system or process being tested does not conform to the expected results. Deviations allow for situations where [/color][COLOR=black][FONT=Verdana]there are no issues or discrepancies within the system, but the test objective cannot be achieved in the prescribed approved way, and so an alternative must be utilised. [/font]