The object for trade that underlies a derivative, such as an option, a warrant or an (structured? In der deutschen Version steht nur "Anlageprodukt") investment product. Underlying assets can be goods or financial instruments (shares, bonds, currencies, indices, etc.). The underlying asset for a share warrant, for example, is the corresponding share; for a DAX Future, the underlying is the DAX itself; for a commodities certificate, it is the commodity.
In addition to physically deliverable underlyings (e.g. commodities, equities, bonds, currencies), there are so-called notional (synthetic) underlyings, such as the Euro-Bund-Futures contract which represents a classic Federal Government Bond. The issuers of certificates often define their own specific underlying assets, e.g. specifically composed indexes or well-defined baskets of stocks. Depending on product conditions, the exact composition of the Underlying may change during the maturity term of the derivative.