Magnetic-core memory (often called simply "core memory") is a class of computer memory devices, which consists of a series of small doughnut-shaped masses of hard ferromagnetic material strung on a wire matrix that can be magnetized in either of two directions. Such computer memory is associated with the central processing unit (CPU) where data is stored for periods of time ranging from a small fraction of a second to days, weeks or even longer before being retrieved for further processing. Magnetic-core memory refers to physical main memory that is internal to the computer. Today, however, other types of main memory devices have replaced magnetic-core memory.

The word "main" is used to distinguish main memory from external (secondary) mass storage devices such as disk drives. Another term for main memory is random access memory (RAM). Before magnetic-core memory, main memory consisted of vacuum tubes. For example, the...