3 Terms and definitions [intro.defs]

3.29[defns.iostream.templates]iostream class templates

⟨library⟩ templates that are declared in header <iosfwd> and take two template arguments
[Note 1:
The arguments are named charT and traits.
The argument charT is a character container class, and the argument traits is a class which defines additional characteristics and functions of the character type represented by charT necessary to implement the iostream class templates.
— end note]