What term best represents the opposite of gain?

The term that best represents the opposite of gain is "loss." In many contexts—whether financial, emotional, or physical—gaining something typically refers to an increase or an advantage, while losing something signifies a decrease or disadvantage. For example, if a business gains profit, a loss would mean a reduction in that profit or a negative financial outcome. This clear dichotomy makes "loss" the most straightforward opposite of "gain."

Other options, while related concepts, do not serve as direct antonyms. "Despair" relates to a state of hopelessness and is not inherently tied to the concept of gain. "Expense" refers to an outflow of money, which can sometimes be thought of as a loss, but it doesn't directly communicate the opposite of gaining something. "Reduction" involves decrease but does not capture the broad context of loss as effectively; for instance, you might reduce expenses without necessarily losing anything. Therefore, "loss" is the most precise choice in representing the opposite of gain.