Magnetic-core memory was the predominant form of random-access computer memory for 20 years (circa 1955-75). It uses tiny magnetic toroids (rings), the cores, through which wires are threaded to write and read information. Each core represents one bit of information. The cores can be magnetized in two different ways (clockwise or counterclockwise) and the bit stored in a core is zero or one depending on that core's magnetization direction. The wires are arranged to allow an individual core to be set to either a "one" or a "zero", and for its magnetization to be changed, by sending appropriate current pulses through selected wires. The process of reading the core causes the core to be reset to a "zero", thus erasing it. This is called destructive readout.
Such memory is often just called core memory, or, informally, core. Although core memory had been superseded by semiconductor memory by the end of the 1970s, memory is still occasionally called "core"; in particular, a file recording the contents of memory after a system error is usually called a core dump