Indifference curve
An indifference curve joins the bundles of goods among which a consumer is equally satisfied.
Offered different combinations of two goods, a person will rank some as better and treat others as equally good. The indifference curve maps the second.
An indifference curve joins all the combinations of two goods among which a consumer is equally satisfied, that is, indifferent. Every bundle on the same curve gives the same level of satisfaction; bundles on higher curves are preferred, those on lower curves less so. It is the basic tool for picturing preferences.
Trading one good for another
Along a single curve, having more of one good is exactly offset by having less of the other, so satisfaction stays constant. The rate at which a consumer will give up one good for more of the other, while remaining equally happy, is the marginal rate of substitution, shown by the slope of the curve. That the curves typically bow inward reflects a sensible idea: the more you have of something, the less of the other good you will sacrifice to get still more of it.
Preferences without measuring happiness
The elegance of indifference curves is that they describe choice without needing to measure satisfaction in absolute units. They require only that a person can rank bundles, prefer this one, that one, or be indifferent, not that happiness be counted. This made it possible to build a coherent theory of consumer choice on rankings alone, sidestepping the awkward problem of quantifying utility.
Choosing the best affordable bundle
Indifference curves come into their own when combined with a budget constraint, the line showing what the consumer can afford. The best affordable bundle is where the budget line just touches the highest reachable indifference curve. At that point the rate at which the consumer is willing to trade the goods equals the rate at which the market lets them, which is the condition for an optimal choice. This is the engine behind the demand curve.
Indifference curves are an abstraction no shopper consciously draws, yet they capture something real: that choice is about trade-offs among bundles, ranked rather than measured. They remain the standard way economists represent what people want and how they choose under the limits of a budget.