One of the styles for writing expressions, with operations in-between the operands (e.g. 6 * 7), as contrasted to postfix (e.g. 6 7 *; operations follow operands, as in RPN) and prefix (e.g. * 6 7; operations come before operands, as in Tcl in general). Infix notation for mathematical expressions is by far the most common, and is in Tcl supported by expr.