Synthetic paper. What is it? It is "paper" that is really sheets of plastic or other substrates. What will they think of next?
My favorite is Yupo. It is sheets of polypropylene. "YUPO Synthetic Paper is 100% recyclable, waterproof and tree-free" per their site. All that is nice, but what I love about it is that it makes colors POP. The piece above is done on Yupo, though I don't think the photo really does it justice. Another piece of mine on Yupo that the photo does justice to is "Chaotic Days" watercolor painting.
If you want a more economical synthetic paper, Mineral Paper is almost as good. Made from rocks. Woot! A sticking point for me is that it doesn't seem to come in weights heavier than 100 lb. Since it's made by Yatsutomo, it is possible that in Japan it is available in more weights.
I was surprised to find that there are many synthetic papers. Most of them seem to be for tags or signs. Yupo or Mineral Paper get my vote for art, though.
Conspicuous note: If you click the Amazon links above and buy anything I will get a small commission. If that bugs you, an Amazon or Google search will find them for you.