blue sheet". This is the Japanese word for a tarp, even if they are not blue. Most are blue here in Japan, though.
I bought one that said it was "small and conveniently folds down to a size easy to carry". It only seats one person, but since I do not really have any friends that want to hang out with me, that is usually big enough, haha. I bought one that was mainly blue, just to be true to the name.