NOR is a function of at least two binary inputs. It returns 0 (true) if at least one of its inputs is 1 and 1 (true) otherwise. Any function of any number of binary inputs can be created as a composition of NOR functions, and a computer can be built entirely out of logic gates that compute the NOR function (called "NOR gates").
NOR also refers to a type of flash memory that is very fast to read randomly. It is called "NOR" because each memory cell looks like a NOR gate. Like RAM and ROM and unlike NAND flash memory, NOR memory is useful for executing a program directly from the chip.