Binary Coded Decimal.
A way of representing integer numbers in a computer. Each decimal digit is stored in 4 bit, so one byte (8 bit) contains two decimal digits.
Also used as the name of a number of character sets (mostly six bits wide) that preceded EBCDIC as the internal representation of alphanumeric Hollerith cards. Generally speaking, the most significant two bits referred to the 'zone' punches 12, 11, and 0, while the least significant two bits referred to the 'numeric' punches 1-9.
See http://www.cwi.nl/~dik/english/codes/80col.html for a discussion.