Honing oil is a liquid, solution or emulsion used to aid in the cutting or grinding of metal, typically by abrasive tools or stones, and may or may not contain oil. It can also be called machining oil, tool oil, cutting fluid, and cutting oil.
In the context of hand blade sharpening, honing oil is used on a sharpening stone to protect the stone, carry away the debris (swarf), and to more efficiently produce a keen edge on a metal blade such as a knife.
 In a machine shop it also carries away excess heat and depending on composition, may prevent unintentional tearing and welding of the metal. Or when used with materials such as soft copper, it may have extra additives to prevent stone loading, or metal deactivators to prevent staining of copper containing alloys.
 To achieve maximum cutting rates and abrasive life with petroleum (mineral) based machining oils when honing difficult materials like stainless steel, a higher level of surface active lubricity agents are combined with sulphur extreme pressure additives.