Define general-purpose registers in a processor?

General-purpose registers are the CPU registers that a programmer can use freely for any purpose. They are not tied to one special job. They can hold data, operands, intermediate results, or memory addresses (as pointers or index values).

Examples are R0 to R15 in ARM, or AX, BX, CX and DX in the Intel 8086. Instructions can name them directly, as in ADD R1, R2. Because the registers are inside the CPU, they are much faster than memory. Using more of them reduces the number of memory accesses and makes the program run faster.