Definition:

Causes/aetiology 

Frank Ocean has done a very cool work.

In capitalist societies wealth represent in form of accumulated commodities. so commodity is the single unit of wealth. we are going to start our study from the basic thing commodity. what is a commodity and what feature makes a thing commodity.
the first thing you have to understand about commodity is that it is something out of us. a human is not a commodity. by saying this we are actually protecting basic human right which is freedom. so the commodity is a thing which at first must satisfy any type of human need. we can say this property of a commodity its use value. so we can say at first step that any thing which has a use value is commodity. but here comes a question that if every use full thing is commodity then nature as a whole is use full and according to human needs like in air we need oxygen and it exists. if we call air a commodity we are wrong because air is not producing wealth and it existed in every economical system.
we need to understand that what is different in between use full things and commodity, the single unite of capitalism. in common worlds we can say a commodity is a use full thing which is sale-able. now we are at the point from where our investigation starts. a thing which can satisfy human need and is sale able called commodity.