The value of the next-best alternative that's given up when a choice is made — every decision has a hidden cost measured not in what you spent, but in what you could have had instead.
Spending $5,000 on a vacation doesn't just cost $5,000 in cash — it also costs whatever that $5,000 could have become if invested instead. That second, invisible cost is the opportunity cost, and it's the reasoning behind phrases like "time is money": every dollar (and every hour) spent one way is a dollar or hour that can't simultaneously be spent another way.
$5,000 spent today has an opportunity cost of roughly $38,000 in 30 years if it could have grown at 7% instead — not because the vacation was a bad idea, but because that's genuinely what was given up to take it. Opportunity cost doesn't say don't spend; it just makes the real trade-off visible.