NOR (not-or) is one type of Boolean logical operation. It accepts two binary digits as input, and produces one binary digit as output. It returns true (1) if both its inputs are false (0), and false if either of its inputs are true.
In computers, the NOR operation may be performed by a dedicated physical device, which is called a logic gate, and implemented as a transistor. A CPU may contain millions of such transistors, each as small as 1 nanometer.