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Probably named after George Atwood.
Etymology from Wiktionary, CC BY-SA 4.0.
Atwood number
Today it means: A dimensionless number used in the study of hydrodynamic instabilities in density stratified flows: A=(ρ₁-ρ₂)/(ρ₁+ρ₂), where ρ₁ is the density of the heavier fluid and ρ₂ is the density of the lighter fluid. Full definition →