Still worn by medieval recreationists, the garment you're looking for is a gambeson. It looks kind of like armour in its own right (because it is) and it looks basically like a tunic. It's typically made of padded leather or cloth. It's worn fairly tight, and might also include points (strings to tie your armour onto) as well. Tap "gambeson" into Google Images and you'll get some decent pictures popping up :)