What’s a non-return valve? I came across a reference to it, but don’t understand what it means.
What’s the context? There are any number of systems that could have a device with that name.
Could they be referring to a backflow preventer, which is used to seal potable water supply from say utility water in a industrial plant? If I remember correctly from my piping design days they were shown as a double check valve on a flow chart.
Broadly, a device that allows flow one way but not the other.
My first thought (without knowing the context) is that it sounds like a backflow-prevention valve. Outdoor sprinkler systems typically have a valve that only lets water flow one way, to make sure that water from outside (which might be contaminated with who-knows-what) doesn’t get pushed into the sprinkler head and back up into the house’s water pipes. I suppose that you might call this one-way valve a “non-return valve.”
If that’s not it, then I have no clue.
They’re also used in the feed to hydronic boilers and fire sprinkler systems to keep the potable water system free of contamination.
Piping, of course. 
The valve goes in your blowpipe so that the bag doesn’t loose pressure when you take a breath.
From here
So it means a one-way valve? That’s what I’ve always called it. Just didn’t recognize the “non-return” phrase.
Thanks, all.
And, I love the Dope. Four useful answers in 20 minutes or less, or your money back! 
Not to mention in your veins, to prevent blood from flowing back towards your tissues and keep it moving towards your heart.
http://www.jobst-usa.com/veinsandvalves.html
Make that five useful answers. 
Also called a check valve.
So it’s a fluidic diode.
…and use of several, as, perhaps an extinguishing device, would wreck da fire (or is that reaching a bit too much, even for me?)