Probably what's called production canvas, which is polyester, as in cheap, and lightweight. I use it off & on and stretching is not a problem. I found it a bit slippery and springy, so it takes a little getting used to keeping it lined up for initial staples.
I have ethical issues with the stuff and will only sell it to somebody after I explain what it is and they agree that it's what they want. What I have a problem with is online print sources that make it sound like you're getting a real canvas. I refuse to call this stuff real canvas.