1.NounA valve, tap or faucet which regulates the flow of liquid or gas through a pipe.
2.NounA main shutoff for water to a home from a municipal supply. Usually these valves exist in pairs, one outside the property boundary and one inside the property boundary.
"If you have a burst pipe, turn the water off at the stopcock."
3.VerbTo control the gas pressure in (an oil well) by keeping the well closed in and the gas confined except during set intervals when the oil is flowed or pumped.