Definition of Utility theory:
Economics concept that although it is impossible to measure the utility derived from a good or service, it is usually possible to rank the alternatives in their order of preference to the consumer. Since this choice is constrained by the price and the income of the consumer, the rational consumer will not spend money on an additional unit of good or service unless its marginal utility is at least equal to or greater than that of a unit of another good or service. Therefore, the price of a good or service is related to its marginal utility and the consumer will rank his or preferences accordingly.
Meaning of Utility theory & Utility theory Definition