WordNet (r) 3.0 (2006):

index register
    n 1: (computer science) a register used to determine the address
         of an operand

The Free On-line Dictionary of Computing (30 December 2018):

index register

    A register found in some CPUs, whose contents
   can be added to the address operand to give the effective
   address.  Incrementing the index register then allows the
   program to access the next location in memory and so on,
   making it very useful for working with arrays or blocks of
   memory.

   Index registers first appeared around April 1949 in the
   Manchester Mark I.  The Mark I's index register's contents
   were simply added to the entire instruction, thus potentially
   changing the opcode (see The story of Mel)!

   (2006-09-20)