Definition of "annulus" []

• A ringlike figure, part, structure, or marking, such as a growth ring on the scale of a fish. (noun)
• A ring or group of thick-walled cells around the sporangia of many ferns that functions in spore release. (noun)
• The ringlike remains of a broken partial veil, found around the stipes of certain mushrooms. (noun)
• Mathematics The figure bounded by and containing the area between two concentric circles. (noun)

American Heritage(R) Dictionary of the English Language, Fifth Edition. Copyright (c) 2011 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.

• "One of the biggest variables is whether the area called the annulus, which is between the inner piping and the outer casing, has sprung an oil leak."
• "One of the biggest variables on the static kill's finality is whether the area called the annulus, which is between the inner piping and the outer casing, has sprung an oil leak."
• "BP Senior Vice President Kent Wells said it's "really a possibility" that cement that engineers pumped in through the top went down into the reservoir, came back up and plugged the annulus, which is between the inner piping and the outer casing."