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

string reduction

   A reduction system where an expression is represented as a
   string of function names, constants and parentheses.  It is
   reduced by replacing parts of the string representing subterms
   by their value.

   It is harder to represent sharing of subexpressions in string
   reduction than in graph reduction.

   (1995-02-06)