A natural gas orifice is a machined hole through which natural gas flows from an internal supply. Although gas orifices are a common feature of many different devices, including furnaces and boilers, the term "natural gas orifice" chiefly pertains to cooking implements, such as barbecue grills or stove top burners.
Cooking implements such as grills have a small metal tube called a valve orifice fitting, which is attached to a fuel supply. When the valve is in an open state, gas streams through the orifice in the center of the fitting and is ignited, providing heat to cook food.
Natural gas orifices are larger than propane orifices, because natural gas supplies are given a lower pressure than similar propane tanks or fuel lines. A larger hole allows natural gas to stream out at a proper volume, despite the lower operating pressure involved.