utility
When you choose chocolate over vanilla, or a bus ride over a long walk, you are revealing that one option does more for you than the other. Economists need a word for that fuzzy thing, the satisfaction, usefulness, or want-fulfilling power you get from a good or service. That word is utility. It is not the same as usefulness in the everyday sense; a diamond ring can carry enormous utility for someone even though it does no practical job at all.
Utility is best understood as a ranking, not a real measurement. Early economists imagined you could count happiness in fixed units called utils, an approach called cardinal utility, but modern economics mostly drops that and uses ordinal utility: it only asks whether you prefer A to B, B to A, or are indifferent. So we do not really say a movie gives you 40 utils; we say you would rather see the movie than read the book, which is all the theory needs. Utility is also personal and subjective: the same cup of coffee can be a delight for one person and a nuisance for another.
This idea anchors the whole theory of consumer choice. Demand curves, budget choices, and the value of goods all rest on the assumption that people act as if they are trying to get the most utility they can from limited money. The honest caveat is that nobody can open up a brain and read off utility; it is a modelling device that helps explain and predict behaviour, not a quantity science can directly measure.
You skip a cheaper sandwich and pay extra for sushi because the sushi gives you more utility today. You cannot say how many utils, only that you prefer it, and that preference is all the model uses.
Utility is the satisfaction a choice delivers, ranked not measured.
Utility is not money and not happiness in any literal, comparable sense. You cannot add your utility to mine, which is why economists are wary of claims that a policy raises total social utility.