Indicates if the specified Serial port is ready.
On the Leonardo, if (Serial) indicates wether or not the USB CDC serial connection is open. For all other instances, including if (Serial1) on the Leonardo, this will always returns true.
This was introduced in Arduino 1.0.1.
Arduino Leonardo specific:
Arduino Mega specific:
boolean : returns true if the specified serial port is available. This will only return false if querying the Leonardo's USB CDC serial connection before it is ready.