A tanker is usually a vehicle carrying large amounts of liquid fuel.

Commercial crude oil supertanker AbQaiq.

The most common use of the word is when referring to a large ship, carrying petroleum products. Apart from pipeline transport, tankers are the only method of transporting large quantities of oil around the world. The supertanker is also the largest type of ship in the world.

Such tankers have several times caused large environmental disasters when sinking close to coastal regions, causing oil spills. See Exxon Valdez, Braer, Prestige, Torrey Canyon for examples of tankers that have caused oil spills.

The largest tanker in the world is the Knock Nevis, also making it the largest ship in the world as well.

Tankers used for liquid fuels are classified according to their capacity to carry oil

• LNG, Liquid Natural Gas, relatively rare carriers for natural gas instead of oil
• VLCC, Very Large Crude Carrier, Over 200,000 metric ton
• SUEZMAX, ship that can pass through the Suez Canal, 125,000 - 200,000 metric ton
• AFRAMAX, Am. Freight Rate Assoc. Size, 80,000 - 125,000 metric ton.
• PANAMAX, ship that can pass through the Panama Canal, 50,000 - 79,000 metric ton
• MR, Medium Range, 38,000 - 50,000 metric ton
• GP, General Purpose, under 38,000 metric ton (possibly outdated name)

In the past the term ULCC, for Ultra Large Crude Carrier of over 300,000 metric ton was also used, and the ????MAX names used to be referred to as varies types of LR, Large Range, such as LR1, LR2, LRC etc.

A tanker is an aircraft used for in-flight refueling. The US Air Force mostly uses converted transport aircraft, such as the KC-135 Stratotanker. The US Navy tends to use smaller aircraft, often converted attack aircraft such as the A-6.

A tanker is any crew member of a main battle tank.

A tanker, in the massive-multiplayer lingo, is a character able to resist or shrug off heavy amounts of damage relatively unharmed. A party with a tanker usually use him as a monster bait -- while he 'takes' the damage, the rest of the party members actually kill the monsters.