Binary-coded decimal (usually written as BCD) is a way of storing arbitrary-length numbers in decimal base on a binary computer. Each decimal digit is stored as four bits, usually two per byte ('packed'), although sometimes only one per byte ('un-packed'). 4-bit values not needed to encode a decimal digit are often used for other symbols, e.g. '-', to indicate a negative number.
On some machines, BCD was supported in the hardware, as a formal data type with instructions which could operate on BCD data (sometimes by an optional Commercial Instruction Set); on other machines, it was done in software, via subroutine packages.