The structure of valid strings in a given language, as described by a grammar. For example, the syntax of a binary number could be expressed as
binary_number = bit [ binary_number ]
bit = "0" | "1"
meaning that a binary number is a bit optionally followed by a binary number and a bit is a literal zero or one digit.
The meaning of the language is given by its semantics.
See also abstract syntax, concrete syntax.