In Microeconomics, this is the rate of which a person will give up one good for another along the same indifference curve. This is a ratio between the two, very similar to the relative price and is equivalent to the opportunity cost of one item in terms of another. This generally has the tendency to diminish as you move down the indifference curve, which is also known as the principle of Diminishing Marginal Rate of Substitution. There are cases when this isn't true, such as when the two items are perfect substitutes, complements, or in terms of total indifference towards the ownership of one of the items on the graph.