means Reduced to liquid by intense heat, as with melted metal, rock, or glass.

from An old past participle of "melt," from Old English "meltan," to dissolve or become liquid. "Melt" later grew a regular ending ("melted"), but "molten" lingered on as a survivor — the antique form kept around for things that glow and flow, like lava and iron. It's a cousin of the same Germanic root that gave Dutch "smelten" and German "schmelzen," and is distantly related to words for softening and dissolving.