A metalanguage is a language (either formal or natural) which is used to make statements about another language (again, either formal or natural).
In the context of formal systems, a metalanguage is a formal language used to specify another formal language.
The object language of a metalanguage is the language described by that metalanguage.
The syntax of a metalanguage is called a metasyntax of the object language of that metalanguage.
A metasymbol is a symbol used in a metalanguage to represent an arbitrary collation in the object language.