Opportunity Cost

Keeping ₹5 lakh in a savings account earning 3% instead of a diversified equity portfolio that might have returned 12% doesn't cost you anything in the sense of a bill - but the opportunity cost is the extra growth you gave up by choosing the safer, lower-return option. Opportunity cost applies well beyond money: time spent on one activity is time not spent on another, and evaluating any choice honestly means asking what the next-best alternative would have delivered, not just whether the chosen option was fine in isolation. It's a comparison against what else was possible, not a judgment that the choice made was wrong.