Ideally, the vacuum chamber should retain the vacuum after the pumps have been turned off. But in real systems, over time, system pressure increases. This increase in pressure can be due to the penetration of wall molecules (Outgassing) or due to leakage and penetration of molecules from outside into the system.
Accordingly, there are two types of leaks, residual leaks and virtual leaks. A residual leak is a real leak due to incomplete sealing, the existence of holes or micro-cracks in the system. A virtual leak is caused by the outgassing of chemicals trapped or adhered to the interior part of a system.