What is a Stem?
Stem (faucet) – A cylinder formed in a long narrow part of a faucet, typically to stop the flow of water.
Ideal Home Garden Explains
The stem of the faucet refers to the device that is attached to the handle that is located in the faucet. When the handle is tightened, the stem touches the valve and therefore stops the flow of water. When the handle is loosened, the stem detracts from the valve allowing water to freely flow. This cylinder device is used to control the flow of water.