Marginal Rate of Substitution (MRS)
Plain definition
The marginal rate of substitution is the rate at which a consumer is willing to trade one good for another while keeping total satisfaction unchanged — how many units of good Y you would give up for one more unit of good X. It equals the absolute slope of the indifference curve.
In economics
MRS is the preference-side exchange rate, mirroring the budget line's market exchange rate: utility is maximized where MRS equals the price ratio. Diminishing MRS — needing ever more Y to compensate for each unit of X given up — is what makes indifference curves convex to the origin.
Example
With 10 apples and 6 oranges, you would give up 2 oranges for one more apple and feel equally satisfied — your MRS of apples for oranges is 2. After several such trades, with apples abundant and oranges scarce, you would give up only half an orange for another apple: diminishing MRS.
MRS is often confused with the budget line's slope. MRS is about willingness — the preference trade-off, the indifference curve's slope. The price ratio is about ability — the market trade-off, the budget line's slope. Choice is optimal when the two are equal, but they are conceptually distinct: one comes from tastes, the other from prices.