Definition: A valve designates a component that is used for blocking or flow control of liquids or gases.

As such, a valve is a fitting and, consequently, a shutoff mechanism. However, certain fittings, such as butterfly valves or ball valves, do not count as valves.
How does a valve work?

Valves, in the precise sense of the word, represent closing mechanisms that are moved parallel to the flow direction of the medium conveyed. If the closing mechanism is pushed against a valve seat, it interrupts the flow. Aside from that, the seal is achieved by means of a rubber-coated closure or a membrane for other shutoff mechanisms

A globe valve body where the flow is redirected