means A fusible metal alloy, typically melted with a hot iron, used to join or seal metal parts and electrical connections.

from From Old French 'soudure,' from the verb 'souder,' to make solid or unite — itself from Latin 'solidare,' to make firm, the same root that gives us 'solid' and 'consolidate.' The buried 'l' that English restored had vanished in French speech, which is why many people still pronounce it 'sodder,' especially in American English: the spelling remembers a Latin ancestor the tongue forgot. So solder is, quite literally, the act of making things solid together.