The idea in one sentence
Opportunity cost is the value of the next-best alternative given up when a choice is made. Every decision that uses a limited resource — money, time, land, labor — necessarily forecloses whatever else that resource could have been used for, and opportunity cost is the name economists give to the value of that best foreclosed alternative. It's arguably the single most foundational concept in economics, because it reframes every decision as a comparison between options rather than an isolated choice made in a vacuum.
Why it's not the same as the price paid
A common confusion: treating opportunity cost as identical to the money spent on something. They're related but distinct. Buying a $15 concert ticket has an explicit cost of $15, but its opportunity cost is whatever else that $15 (and the evening's time) would have been worth — a different night out, an hour of paid work, or simply the value of having that money available for something else later. If the $15 would otherwise have been spent on nothing more valuable, the opportunity cost is close to the sticker price. If it displaced something more valuable, the opportunity cost exceeds the sticker price. The dollar amount spent and the opportunity cost only coincide when the next-best alternative happens to have exactly that value — which isn't guaranteed.
Time has an opportunity cost too, without any money changing hands
Opportunity cost applies to any limited resource, not just money, which is why it explains decisions with no purchase involved at all. Spending three hours watching a movie has an opportunity cost equal to whatever else those three hours could have produced — studying, working, resting, spending time with someone. No money changes hands in this example, but the choice is still economically real: the three hours are gone regardless of what they were spent on, and choosing the movie means not choosing whatever the next-best use of that time would have been.
This is part of why opportunity cost is considered a more complete way to think about cost than price alone — many of the most important tradeoffs people actually make (how to spend time, which of two job offers to take, whether to attend college instead of working) don't reduce cleanly to a dollar figure, but they're still real tradeoffs with a genuine opportunity cost.
A worked example: choosing a college major
Consider a student choosing between two college majors that both take four years to complete. The tuition cost might be identical either way, so a comparison based only on money spent would show no difference between the two options. But if one major typically leads to a starting salary of $70,000 and the other to $50,000, choosing the lower-paying major carries a real opportunity cost — the $20,000-per-year difference in expected income — even though no additional dollar was ever spent to "cause" that cost. Opportunity cost captures this kind of foregone value that a simple spending comparison would miss entirely.
Why "there's no such thing as a free lunch" is really about this
The common saying "there's no such thing as a free lunch" is a compressed way of stating the opportunity cost principle: even something offered at zero explicit price still has a cost, because accepting it uses time, attention, or some other resource that then can't be used for anything else. A "free" seminar still costs the attendee whatever else that hour could have been spent on. The saying isn't claiming nothing is ever literally free of charge — it's pointing out that opportunity cost exists independently of whether money is involved.
Using it to actually evaluate a decision
The practical use of opportunity cost isn't just naming the concept — it's using it as a discipline for comparing options honestly. Before a decision, identifying the realistic next-best alternative (not a fantasy alternative, but what would genuinely be chosen instead) and asking whether the option under consideration is actually worth more than that alternative turns a vague "is this a good idea?" into a concrete comparison. Decisions that look reasonable in isolation sometimes look much less appealing once the actual foregone alternative is named specifically, rather than left as a vague, unspecified "something else."
Why economists treat this as foundational
Opportunity cost is often introduced as the very first concept in an economics course because nearly every other concept in the field — supply and demand, comparative advantage, cost-benefit analysis — builds on the underlying idea that resources are limited and every choice therefore forecloses some alternative. Understanding opportunity cost well enough to identify it in unfamiliar situations (not just the standard textbook examples) is what makes the rest of economic reasoning click into place as one coherent way of thinking, rather than a set of separate, unconnected rules.