Some price hikes barely dent sales. Others wipe them out

Raise the price of insulin or gasoline and people mostly keep buying it anyway — demand is 'inelastic' because there's no good substitute. Raise the price of a specific brand of soda by the same percentage and sales can collapse, because shoppers just grab a different can. The ratio between the two reactions is what economists call elasticity, and it decides who actually bears a tax or a price increase.

— Alfred Marshall, Notes on the Elasticity of Demand — Principles of Economics, 1890