• oil
• 1

  fuel

   [uncountable] a smooth thick liquid that is used to make machines run easily or is burned to produce heat:
• Check the oil level in your car every week.
• The heating system runs on oil.
• 2

  natural substance under the ground

   [uncountable] the thick, dark liquid from under the ground from which petrol is produced [↪ crude]:
• a rise in the price of oil
• oil industry/company/production etc
• the importance of protecting our oil supplies
• an oil refinery (=factory where oil is made purer)
• 3

  liquid from plants

   [uncountable and countable]a smooth, thick liquid made from plants or some animals, used especially in cooking or for making beauty products:
• cooking oil
• olive/vegetable/sunflower etc oil
• coconut oil shampoo
• Fish oils are supposed to help relieve arthritis.
• ➔ castor oil, cod-liver oil, linseed oil
• 4

  paint

   oils

   [plural] paints that contain oil [= oil paints]
• in oils
• I usually paint in oils (=using oils).

• ➔ burn the midnight oil

   at burn1 (20)

   ; ➔ pour oil on troubled waters

   at pour (8)

Definition from the Longman Dictionary of Contemporary English
Advanced Learner's Dictionary.