@@ -- a binary operator

Ways to use symbol @@ :

Expression @@ Expression -- see Expression -- the class of all expressions
Expression @@ Holder -- see Expression -- the class of all expressions
Expression @@ Thing -- see Expression -- the class of all expressions
Holder @@ Expression -- see Expression -- the class of all expressions
Holder @@ Holder -- see Expression -- the class of all expressions
Thing @@ Expression -- see Expression -- the class of all expressions
Function @@ Function -- composition of functions

For the programmer

The object @@ is a keyword.

This operator may be used as a binary operator in an expression like x@@y. The user may install binary methods for handling such expressions with code such as

         X @@ Y := (x,y) -> ...

where X is the class of x and Y is the class of y.