The Collaborative International Dictionary of English v.0.48:

infix notation \in"fix no*ta"tion\, n. (Computers)
   A notation for commands or instructions in which the operator
   is inserted between the terms on which it operates.
   Contrasted with prefix notation and postfix notation.

WordNet (r) 3.0 (2006):

infix notation
    n 1: a notation for forming mathematical expressions using
         parentheses and governed by rules of operator precedence;
         operators are dispersed among the operands

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

infix notation
infix syntax

    One of the possible orderings of functions and
   operands: in infix notation the functions are placed between
   their operands, such as "1+2".  Although infix notation is
   limited to binary functions most languages mix infix notation
   with prefix or postfix notation, as a form of syntactic
   sugar.

   (1997-01-17)