Infix

Infix[f[e1,e2,…]] prints with f[e1,e2,…] given in default infix form: e1~f~e2~f~e3…. 

Infix[expr,h] prints with arguments separated by h: e1 h e2 h e3…. 

Please visit the official Wolfram Language Reference for more details and examples on core symbols.