I use clear acrylic/ perspex primarily because its free for me. As ramsayii states they are the same thing. There are numerous brands and perspex has become the generic.
Good things are it can be drilled, edges can be polished and it can be glued. It can be heat bent.
Downside is it marks easily and translucence diminishes over a long period. The better the quality the longer it lasts. I have had sheets standing in the backyard for years and they are still good.
If you have a problem with it flexing then it is to thin. I use mainly 10 or 12 mm Uv resistant.
Great tip on the UV resistant, I guess this helps with clarity over a long period of time.