basaltic lava having a rough surface.
Also, a'a.
Compare pahoehoe.

1855–60; < Hawaiian ʿaʿā

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Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
aa1 (ˈɑːɑː)
a volcanic rock consisting of angular blocks of lava with a very rough surface

abbreviation for

abbreviation for
1.  Alcoholics Anonymous
2.  anti-aircraft
3.  Architectural Association
4.  (in Britain) Automobile Association
5.  (in Britain) Advertising Association
symbol for
6.  (in Britain, formerly) a film that may not be shown publicly to a child under fourteen

Collins English Dictionary - Complete & Unabridged 10th Edition
2009 © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009
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Word Origin & History

abbreviation of Alcoholics Anonymous, attested by 1941, Amer.Eng. The group name was the title of a book published in 1938 by the founder, Bill W.
Online Etymology Dictionary, © 2010 Douglas Harper
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American Heritage
Medical Dictionary

AA abbr.

  1. Alcoholics Anonymous

  2. amino acid

  3. aminoacyl

The American Heritage® Stedman's Medical Dictionary
Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.
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American Heritage
Science Dictionary
aa   (ä'ä)  Pronunciation Key 
A type of lava having a rough, jagged surface. It is relatively slow moving in its molten state, advancing in the form of massive blocks with fissured and angular surfaces that ride on a viscous interior. The blocks range in size between the size of a football and the size of a house. See Note at pahoehoe.
The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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American Heritage
Abbreviations & Acronyms
  1. Afar (language)

  2. author's alteration

  1. Academy of Aphasia

  2. Acts of the Apostles

  3. administrative assistant

  4. adverse action

  5. Alcoa Inc.

  6. Alcoholics Anonymous

  7. Alzheimer's Association

  8. antiaircraft

  9. Armed Forces Americas (except Canada)

  10. Associate in Arts

  11. atomic absorption

The American Heritage® Abbreviations Dictionary, Third Edition
Copyright © 2005 by Houghton Mifflin Company.
Published by Houghton Mifflin Company. All rights reserved.
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