I’ve been told that I haven’t defined “readymade”. True enough.
It’s a kind of “nonretinal” art invented by Duchamp. He would pick an item — generally something mass produced — and declare it to be art. He’d put it on a plinth and even sign it, despite having had nothing else to do with it other than picking it out.
Famous examples include a bottle rack, a bicycle wheel and front fork attached to the top of a wooden stool, and the infamous urinal (named “Fountain” and signed “R. Mutt”.)
If you feel that maybe a “readymade” might not be art, you’re not alone. Duchamp was clearly pushing everyone’s buttons with this particular kind of “work”, and there are arguments about the concept to this day. That, I’m sure, Duchamp would love.