A metallic alloy used to seal joints. In plumbing solder is used to seal metal pipes, in electronics it is used to electrically and mechanically join components together. For example, this is what is used to affix components to an electronic circuit board. Plumbing solder and electronic solder cannot be used interchangeably as they are comprised of different materials. At room temperature solder is a solid metal, but when heated by a “soldering iron” or some other heat source it becomes liquid. In fact, it very suddenly becomes liquid. It doesn’t go through much of a “soft” stage at all, which is one of its desirable properties. To solder (verb) is the process of heating solder and using it to join materials. Once it cools it very quickly becomes a solid metal again and provides (when done correctly) a strong electrical and mechanical joint.