There are 2 main definitions of aa in English:

aa1

Line breaks: aa
Pronunciation: /ˈɑːɑː
 
/

noun

[mass noun] Geology
Basaltic lava forming very rough, jagged masses with a light frothy texture. Often contrasted with pahoehoe.
More example sentences
  • Roads of black ash traverse it, winding through the otherwise untraversable aa.
  • A serpentine flow meanders through formations of smooth pahoehoe lava and rough aa lava, the same forms common in basalt lavas.
  • Very different from jagged aa, pahoehoe is the other general texture of newly solidified lava.

Origin

mid 19th century: from Hawaiian 'a-'a.

Definition of aa in: