Infix notation is one of the 4 possible ways to describe a program. It is not as powerful as the others, so people usually add parenthesis, which makes it a combined notation (Infix-Surround).
Here is an example of pure infix:
1 + 2
And a way to encode * + 2 2 4 in infix-surround:
(2 + 2) * 4