register

A Storage-Class Specifier that defines a local variables to be stored in a register instead of RAM. This means that the variable has a maximum size equal to the register size (usually one word) and may not have the unary '&' operator applied to it (as it does not have a memory location).

You should only use register for variables that require quick access such as counters. Note that defining a variable as a register type, does not mean that the variable will be stored in a register. It means that it MIGHT be stored in a register, depending on the hardware and implementation restrictions.