Define consumer's surplus?

Consumer's surplus is the difference between the maximum amount a consumer is willing to pay for a good and the amount they actually pay for it.

1. Origin and basis: the concept was introduced by A.J. Dupuit and developed by Alfred Marshall, and it rests on the law of diminishing marginal utility — since a consumer values earlier units more than the price paid, the extra satisfaction over price is the surplus. It is expressed as Consumer's Surplus = Total Utility − Total Amount Spent, and graphically it is the area below the demand curve and above the price line.
2. Example: if you are willing to pay Rs. 25,000 for a phone but its market price is only Rs. 20,000, your consumer's surplus is Rs. 5,000.