Aa

Part of speech: noun

Definitions

1. A rough, jagged kind of basaltic lava whose surface breaks into sharp, clinker-like blocks as it cools
2. Volcanic rock with a crumbly, spiky surface that forms when fast-moving basalt lava cools into broken fragments
3. The type of lava flow that hardens into a rugged, rubbly crust of sharp, loose clinkers rather than a smooth sheet

Etymology: "Aa" is a noun that stands for something rather than a word with a long English pedigree: in the Translingual section of dictionaries it appears as an abbreviation, and it is also used as a contraction, so its history is less a single lineage than a set of uses. Compare the abbreviation "pp." (pages, or "per procurationem" in letters), which likewise packs a whole Latin or English phrase into two letters. "Aa" belongs to the same family of short forms that exist only to save space in print and writing, and it is the kind of symbol that gains its meaning from the context around it rather than from any root it carries.