v.1. To melt from a solid into a liquid: As ice cubes melt down, the water level in the glass remains constant.
2. To cause something to melt from a solid into a liquid: The sun melted the snowman down into a puddle. I melted down the ice on the window with a hair dryer.
3. To overheat severely and melt, resulting in the escape of radiation from a nuclear reactor core: The nuclear reactor melted down, and thousands of people had to evacuate the area.
4. Slang To have an emotional breakdown: I was dealing with a lot of stress, and I melted down on the subway when it stopped between stations.