Valve leakage refers to flow through a valve which is set in the 'off' state.

The importance of valve leakage depends on what the valve is controlling; a dripping tap is less significant than a leak from a six-inch pipe carrying high-pressure radioactive steam.

ANSI

Ansi

Ansi is a village in Kaarma Parish, Saare County, on the island of Saaremaa, Estonia....

 specifies six different leakage classes, with 'leakage' defined in terms of the full open valve capacity

• Class I, or 'dust-tight' valves, are intended to work but have not been tested
• Class II valves have no more than 0.5% leakage with 50psi (or less if operating pressure is less) of air pressure at the operating temperature
  Operating temperature
  An operating temperature is the temperature at which an electrical or mechanical device operates. The device will operate effectively within a specified temperature range which varies based on the device function and application context, and ranges from the minimum operating temperature to the...
  
  
• Class III valves have no more than 0.1% leakage under those conditions; this may require soft valve seats, or lapped metal surfaces
• Class IV valves have no more than 0.01% leakage under those conditions; this tends to require multiple graphite piston rings or a single Teflon piston ring, and lapped metal seats.
• Class V valves leak less than 5*10^-12 cubic metres, per second, per bar of pressure differential, per millimetre of port diameter, of water when tested at the service pressure.

Class VI valves are slightly different, in that they have soft seats, and are required (at 50psi or operating pressure whichever is less) to have less than a specified leakage rate in millilitres of air per minute:

1 inch	0.15 ml/min
1.5 inch	0.30 ml/min
2 inch	0.45 ml/min
2.5 inch	0.60 ml/min
3 inch	0.90 ml/min
4 inch	1.70 ml/min
6 inch	4.00 ml/min
8 inch	6.75 ml/min
10 inch	9 ml/min
12 inch	11.5 ml/min

(data from http://www.svf.net/pdf/Class_VI.pdf)