Liquid fuel
From Kerbal Space Program Wiki
Liquid fuel is a resource used to power various engines. It is commonly stored in liquid fuel tanks or fuselages.
Containers
Liquid fuel can be pumped from a fuel tank to another fuel tank or engine connected via an FTX-2 External Fuel Duct. Fuel ducts aren't necessary when the components are adjacent. Fuselages only contain liquid fuel and liquid fuel tanks also contain oxidizer in the correct ratio.
Associated engines
Jet engines need both liquid fuel and intake air to run. Liquid fuelled rocket engines run on a mixture of liquid Fuel and oxidizer