Error-Correcting code (ECC) memory
A type of memory that can automatically detect and correct the most common kinds of internal data corruption.
Electrical or magnetic interference inside a computer system can cause bits of memory to flip to the opposite state. ECC memory can protect memory from single-bit errors, so that the data that is read from each word is always the same as the data that had been written to it, even if one or more bits actually stored have been flipped to the wrong state.
ECC memory is used in most computers where data corruption cannot be tolerated under any circumstances, like scientific or financial computing.
Further information on ECC memory can be found on on Wikipedia.