A commodity as defined by Marx is an "external object, a thing which through its qualities satisfies human needs of whatever kind" (Marx, Capital) and is then exchanged for something else. When Marx speaks of commodities, he is particularly concerned with the "physical properties of the commodity", which he associates closely with the use-value of an object.
"He who satisfies his own need with the product of his own labour admittedly creates use-values, but not commodities. In order to produce the latter, he must not only produce use-values for himself, but use-values for others, social use-values". Commodities, therefore, "possess a double form, i.e. natural form and value form". The physical body of the commodity is made up of 1) the material provided by nature (e.g. linen, gold, etc.); and 2) the economic production expended to create it (see Marx, Capital). What matters is that something be exchanged for the thing, and the thing is a commodity.
In economics, a commodity is an economic good, usually a resource, that has full or substantial fungibility: that is, the market treats instances of the good as equivalent or nearly so with no regard to who produced them.