means A device that controls the flow of liquid, gas, or other matter through a passage, typically allowing it to move in one direction or shutting it off entirely.

from From Latin 'valva,' meaning one of the leaves of a folding or double door — the swinging panels you'd push through. The plural 'valvae' gave English 'valve,' which first kept that 'door-leaf' sense before settling, by the time of plumbing and anatomy, onto the small gates that open and close inside pipes, hearts, and engines. The image survives nicely: every valve is still a little door deciding what passes.