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

instruction register

    (IR) The part of a central processing unit
   (CPU) control unit that holds the machine instruction
   currently being executed.  The CPU's fetch-execute cycle
   loads instructions from memory into the instruction
   register.  The IR in turn drives the instruction decoding
   logic that determines what operation teh CPU should perform on
   what data.

   Modern processors have an instruction pipeline to minimise
   the time the control unit is waiting for instructions and data
   to be fetched from memory.  This may distrubute the job of the
   IR over several registers in the pipeline.

   (2018-08-31)