I

Inhibitor – Any substance which slows or prevents chemical reaction or corrosion.

Interfacial Tension (I.F.T.) – The energy per unit area present at the boundary of two immiscible liquids. It is commonly measured as the force per unit length necessary to draw a thin wire or ring through the interface.

Intermediate Base Crude – See Mixed Base Crude.

ISO – International Standards Organization, sets viscosity reference scales.