Indifference Curve
Plain definition
An indifference curve connects all the bundles of two goods that give a consumer exactly the same level of satisfaction. The consumer is indifferent between any two points on the same curve — each is equally preferred.
In economics
Indifference curves map preferences: they slope downward (more of one good compensates for less of the other), are convex to the origin (diminishing marginal rate of substitution), never cross, and lie farther from the origin at higher utility levels. The consumer's optimum is where the highest attainable curve just touches the budget line.
Example
You are equally happy with (4 samosas, 1 chai) and (2 samosas, 3 chais) — both bundles sit on one indifference curve. A bundle of (5 samosas, 4 chais) lies on a higher curve: strictly preferred, though perhaps unaffordable.
Students often let indifference curves cross or slope upward. Crossing curves would mean one bundle is simultaneously on two utility levels — a logical contradiction, so it cannot happen. An upward slope would mean the consumer is indifferent between a bundle and one with more of both goods, violating the principle that more is better. Downward-sloping, non-crossing, convex: always.