Technically, a leak is a hole or porosity in an enclosure capable of passing a fluid from the higher pressure side to the lower pressure side. Leaks are often conceived of being simply a round hole, however, this is almost never the case. A leak normally has an involved geometry sometimes extending quite a distance from beginning to end. As a result, leakage repair may require locating both the start and end of the leak. Leaks do not always operate in the same manner.
Leaks tend to grow over time and they tend to operate differently under different conditions.
For instance, many leaks are open only intermittently due to temporary clogging by water vapor or unusual pressures. High temperature or vacuum leaks often disappear when operating conditions are removed. As a result, simulating actual operating conditions is best when testing for critical leaks. The word leakage refers to the flow of a fluid through a leak without regard to the physical size or shape of the hole. Leakage typically occurs as a result of a pressure differential across the hole.