I can't even find a picture of it because I don't know what to search for. It's a circular rubber foot that attaches typically to glass, by creating a little vacuum when you press it onto the surface. What's it called?
It's called a suction cup. According to Wikipedia:
A suction cup, also known as a sucker, is a device or object that uses the negative fluid pressure of air or water to adhere to nonporous surfaces, creating a partial vacuum.
Attribution: "Suction Cup." Wikipedia. February 28, 2018. Accessed April 05, 2018. https://en.wikipedia.org/wiki/Suction_cup.