Economic decisions often involve studying the effects of small changes on costs, revenue, and profit. Economists use "marginal" to describe the change from a one-unit adjustment. For example, marginal cost is the cost change from producing one more item. Marginal revenue is the revenue change from selling one more item. In economics, the term "marginal" is like the derivative in math, showing the rate of change. This helps in situations where exact data is hard to get, but close estimates are enough for decision-making.