A commodity is any useful or valuable thing, especially something that is bought and sold. Grain, vegetables, and precious metals are commodities, but so are personal qualities that can be used to make money: Originality and imagination are rare commodities in the film business.
Farmers closely follow the commodities' markets, where the future value of the crops they grow are traded, and on any given day assigned a different value. Commodity descends from Middle English commodite "benefit, profit," from Old French, from Latin commoditās "fitness," from commodus "convenient, appropriate," from the prefix com- "with" plus modus "measure."