a symbol in a mathematical or logical expression that may be replaced by the name of any element of a set [n -S]
A significant zero in the decimal representation of a number
A symbol or piece of text used in a mathematical expression or in an instruction in a computer program to denote a missing quantity or operator
An element of a sentence that is required by syntactic constraints but carries little or no semantic information, for example the word it as a subject in it is a pity that she left, where the true subject is that she left