What is a formal parameter?

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There are formal and actual parameters:

void foo(int arg); //arg is a formal parameter

int main()
{
    int val = 1;
    foo(val);  //val is an actual parameter
}

From C++ Standard:

1.3.1 formal parameter (parameter)

an object or reference declared as part of a function declaration or
definition, or in the catch clause of an exception handler, that
acquires a value on entry to the function or handler; an identifier
from the comma-separated list bounded by the parentheses immediately
following the macro name in a function-like macro definition; or a
template-parameter. Parameters are also known as formal arguments or
formal parameters.

1.3.10 actual parameter (argument)

an expression in the comma-separated list bounded by the parentheses
in a function call expression, a sequence of preprocessing tokens in
the comma-separated list bounded by the parentheses in a function-like
macro invocation, the operand of throw, or an expression, type-id or
template-name in the comma-separated list bounded by the angle
brackets in a template instantiation. Also known as an actual argument
or actual parameter.

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