Wikipedia

A commodity is anything for which there is demand, but which is supplied without qualitative differentiation across a market. In other words, copper is copper. Rice is rice. Stereos, on the other hand, have many levels of quality. And, the better a stereo is, the more it will cost. Whereas, the price of copper is universal, and fluctuates daily based on global supply and demand.
One of the characteristics of a commodity good is that its price is determined as a function of its market as a whole. Well-established physical commodities have actively traded spot and derivative markets. Generally, these are basic resources and agricultural products such as iron ore, crude oil, coal, ethanol, sugar, coffee beans, soybeans, aluminum, rice, wheat, gold and silver.
Commoditization occurs as a goods or services market loses differentiation across its supply base, often by the diffusion of the intellectual capital necessary to acquire or produce it efficiently. As such, goods that formerly carried premium margins for market participants have become commodities, such as generic pharmaceuticals and silicon chips.

In the original and simplified sense, commodities were things of value, of uniform quality, that were produced in large quantities by many different producers; the items from each different producer are considered equivalent. It is the contract and this underlying standard that define the commodity, not any quality inherent in the product.

Investopedia

1. A basic good used in commerce that is interchangeable with other commodities of the same type. Commodities are most often used as inputs in the production of other goods or services. The quality of a given commodity may differ slightly, but it is essentially uniform across producers. When they are traded on an exchange, commodities must also meet specified minimum standards, also known as a basis grade.
2. Any good exchanged during commerce, which includes goods traded on a commodity exchange.
Investopedia Says:
1. The basic idea is that there is little differentiation between a commodity coming from one producer and the same commodity from another producer - a barrel of oil is basically the same product, regardless of the producer. Compare this to, say, electronics, where the quality and features of a given product will be completely different depending on the producer. Some traditional examples of commodities include grains, gold, beef, oil and natural gas. More recently, the definition has expanded to include financial products such as foreign currencies and indexes. Technological advances have also led to new types of commodities being exchanged in the marketplace: for example, cell phone minutes and bandwidth.
2. The sale and purchase of commodities is usually carried out through futures contracts on exchanges that standardize the quantity and minimum quality of the commodity being traded. For example, the Chicago Board of Trade stipulates that one wheat contract is for 5,000 bushels and also states what grades of wheat (e.g. No. 2 Northern Spring) can be used to satisfy the contract.

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