knob, handle or valve

[curiousmarcus]

Do you call the thing you turn a knob, handle, or valve?

 

[GoesStation]

When it's round, I'd call it a knob. Turning it opens or closes the valve it's attached to.

 

[SoothingDave]

It's a faucet.

 

[tedmc]

Those are knobs of a mixer tap (BrE)
The valves which operates the tap are housed within.

 

[Raymott]

No one I know would call it anything other than a tap. A plumber might make a differentiation for each part, but I don't know any plumbers.

 

[tedmc]

They are not knobs in BrE. The whole thing is a mixer tap. The roundish things are the hot and cold taps.

The whole unit is called a tap. The part which you turn the water on is called a handle. Handles come in all shapes. There are the long lever handles called "tap levers". There are the the classic cross handles and rounded knob-shaped handles(knob handles?).

http://www.notjusttaps.co.uk/range-spares-fittings.htm

 

[tedmc]

I am sure OP knows what a tap is but that wasn't the question.
The question was about a specific component part of a tap.

 

[Rover_KE]

And the OP now knows what native speakers call said component.

 

[Skrej]

In AmE all three options would work colloquially, although formally only handle or knob would work.

While the valve is technically the internal component which regulates the water flow, it's not uncommon to refer to the handle as a valve, since turning it regulates the valve.

You'll note a lot of overlap in the pictures for the following searches:
replacement faucet knobs
replacement faucet handles

The search for 'valves' tends to return results of the internal mechanism however.
replacement faucet valves