A relative price is the price of a commodity such as a good or service in terms of another; i.e., the ratio of two prices. A relative price may be expressed in terms of a ratio between any two prices or the ratio between the price of one particular good and a weighted average of all other goods available in the market. A relative price is an opportunity cost. Microeconomics can be seen as the study of how economic agents react to changes in relative prices.
In the demand equation Q = f(P) (in which Q is the number of units of a good or service demanded), P is the relative price of the good or service rather than the nominal price. It is the change in a relative price that prompts a change in demand. For example, if all prices rise by 10% there is no change in any relative prices, so if consumers’ nominal income and wealth also go up by 10% leaving real income and real wealth unchanged, then demand for each good or service will be unaffected. But if the price of a particular good goes up by, say, 2% while the prices of the other goods and services go down enough that the overall price level is unchanged, then the relative price of the particular good has increased while purchasing power has been unaffected, so the demand for the good will go down.
Often inflation makes it difficult for economic agents to immediately distinguish increases in the price of a good which are due to relative price changes from changes in the price which are due to inflation of prices in general. This situation can lead to allocative inefficiency, and is one of the negative effects of inflation.