A BCD adder is a digital circuit used to perform addition on numbers represented in Binary-Coded Decimal (BCD) format. BCD is a way of encoding decimal digits where each digit is represented by its own binary sequence. For example, the decimal digit 5 is represented as 0101 in BCD. The BCD adder takes two BCD inputs, adds them, and produces a BCD output. When the sum of two BCD digits exceeds 9 (1001 in binary), the BCD adder must correct the result by adding 6 (0110 in binary) to the output. This adjustment ensures that the output remains a valid BCD representation, allowing for accurate decimal calculations.