Molten describes an object that's reduced to liquid form by heating. You're probably familiar with lava, the molten rock that explodes out of a volcano.
Molten originated from the Old English verb meltian, meaning "become liquid." It takes incredibly high temperatures to get an object that was once rock solid to be transformed into a liquid state. Something much safer than lava has made the term molten popular — molten chocolate cake. This cake has a center that's filled with hot, gooey pudding-like chocolate that oozes out. This is probably the only molten thing you'd ever want to touch.