Any chemical not on the Toxic Substances Control Act Inventory is considered a new chemical substance. An existing chemical can also be considered new if a 'significant new use' of the chemical could impact human health or the environment.
It's critical to determine if a chemical is on the Inventory before manufacturing (and importing) a chemical substance. Section 5 of TSCA requires manufacturers of new chemical substances for non-exempt commercial purposes to provide the EPA with a PMN at least 90 days in advance.