drying oil

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun An organic oil, such as linseed oil, used as a binder in paints and varnishes. It dries into a tough elastic layer when applied in a thin film and exposed to air.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun An oil that hardens to a tough, solid film after a period of exposure to air.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun an oil that hardens in air due to oxidation and is often used as a paint or varnish base