Definition
A substance under REACH is a chemical element or compound, including its additives and impurities. This is the base unit that gets registered. Registration, evaluation and authorisation under the regulation all attach to the substance.
Substances are identified by markers such as the EC number, which keeps the registered identity consistent across the supply chain. The full framework is set out in REACH.
Note: general educational information, not legal advice. Check the official source before relying on it.