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

loop combination
loop fusion

   A program transformation where the bodies of two loops are
   merged into one thus reducing the overhead of manipulating and
   testing the control variable and branching.  Further
   optimisation of the merged code may then become possible.

   In horizontal loop combination the bodies of the loops are
   largely independent so only the loop overhead is saved.
   Vertical loop combination applies where the results of the
   first loop are used by the second.  Combining the two allows
   the intermediate results to be used immediately (in
   registers) rather than requiring them to be stored in an
   array.

   The functional equivalent of horizontal and vertical loop
   combination are tupling and fusion.