Something physical or tangible that may be referenced by a contract or trade

Detail

Financial derivatives are completely cash-based and usually have no physical underliers

Energy derivatives usually have at least one physical underlier, which may be a commodity,

e.g. coal

or a something related to a commodity

e.g. storage

The underlier acts as a bridge to the physical world - and usually as a set of reference prices for price-setting and valuation

We need to emphasize that nearly all energy trades have at least one physical underlier - few of them actually involve the delivery of energy commodities