WordNet (r) 3.0 (2006):

prefix notation
    n 1: a parenthesis-free notation for forming mathematical
         expressions in which each operator precedes its operands
         [syn: prefix notation, Lukasiewicz notation, Polish
         notation]

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

prefix notation
prefix syntax

    (Or "prefix syntax") One of the possible orderings
   of functions and operands: in prefix notation the function
   precedes all its operands.  For example, what may normally be
   written as "1+2" becomes "(+ 1 2)".  A few languages (e.g.,
   lisp) have strictly prefix syntax, many more employ prefix
   notation in combination with infix notation.

   The opposite, postfix notation, is somewhat rarer.

   (2014-07-08)