A metal alloy of low melting point, necessarily lower than that of the metals it bonds, used to provide a low-heat welding bead between two conjoined metal surfaces. Normally applied with a liquid flux to help it delver an even weld. usually bought in the form of a continuous wound coil. Apparently the "l" is silent in American English ("sohder") but is pronounced in British English. Encountered in the Series Nine episode "The Solder Excursion Diversion".