Opportunity cost
Also said as Trade-off, Tempo cost, What you gave up
What a choice really costs you is the best other thing you could have done with it. The gold you spent on a farm is a soldier you don't have when the raid comes.
It is what separates a real choice from a shopping list. If you can have both, or one is plainly free, there is nothing to think about. Well-designed games make you feel the option you turned down.
Say it
“Upgrades are free choices right now, because everyone can afford all of them by turn ten. Make each one cost a full turn of production, so that upgrading means not building.”
Where you’ve seen it
StarCraft asks it over and over in the first minutes: another worker pays off later, but it is a marine you don't have if the enemy attacks early. A greedy build wins if you survive it.
Not to be confused with price
The price is what the tag says. The opportunity cost is what else that price would have bought, and it changes with the moment: gold is worth more one turn before an attack.