Definition of "aa"

aa¹

/ˈɑː.ɑː/

noun
uncountable
1. (volcanology) A form of lava flow associated with Hawaiian-type volcanoes, consisting of basaltic rock, usually dark-colored with a jagged and loose, clinkery surface. Compare pahoehoe.
  Quotations
  We...saw ‘pahoihoi’ or solid lava forming, and also ‘aa’ or clinkers.
  1859, R. C. Haskell, American journal of science and arts, series XXVIII
  Cooling and solidification frequently takes a different course [...] in lava flows, producing the clinker-like ‘aa’ lava.
  1944, Charles A. Cotton, Volcanoes as landscape forms
  Both pahoehoe and aa lava flows are common on the upper slopes of Mauna Loa with a preponderance of aa flows found at the lower elevations.
  1981, Hilo Lava Flood Control: Environmental Impact Statement, page 194

aa²

noun
1. Initialism of acetic acid.
2. Initialism of acting age.
3. (medicine) Initialism of alveolar-arterial.
4. Initialism of aminoacetone.
5. Initialism of amino acid.
6. Initialism of approximate absolute.
7. Initialism of arachidonic acid.
8. Initialism of armature accelerator.
9. Initialism of ascending aorta.
10. Initialism of atomic absorption.
11. Initialism of author's alteration.
12. Initialism of average audience.
adjective
not comparable
1. Initialism of arctic-alpine.
2. Initialism of always afloat.

aa³

noun
plural only
1. Abbreviation of adjectives.
2. Abbreviation of arteries.

aa⁴

adjective
1. Abbreviation of ana (“of each, equal parts”).