Steve is correct
If you are talking about the rubber hose that goes from the top of the breather to the valve cover, it is the breather hose. Note that there is also a clear plastic tube that goes into the breather hose and goes down below the breather. Be carefull that you don't break this plastic tube, with age it becomes yellow and very brittle. I know, I broke mine. The good news is that you can patch it with a section of aquarium tubing.