1) Digital register 
An index register in a computer`s CPU is a processor register used for modifying operand addresses during the run of a program, typically for doing vector/array operations. The contents of an index register is added to (in some cases subtracted from) an immediate address (one that is part of the instruction itself) to form the `effective` addre......
[n] - (computer science) a register used to determine the address of an operand
A microprocessor register that holds part of or all the effective address used by an instruction. See also: Effective Address, Register.
noun (computer science) a register used to determine the address of an operand