A tracksuit. This term is very commonly used in Australia.
I walked to the shop in my old trackie-daks and thongs.
trackie-daks are the bottom half of the tracksuit, not the whole. the whole tracksuit is the trackie; the pants are the trackie-daks. but Random's example is unsurpassable :)
they wouldn't let me into the Rissole wearing me trackie-daks and thongs.