Return on investment (ROI) measures the gain or loss generated on an investment relative to the amount of money invested. ROI is usually expressed as a. Definition of return on investment (ROI): The earning power of assets measured as the ratio of the net income (profit less depreciation) to the average capital. Return on investment, or ROI, is the most common profitability ratio. There are several ways to determine ROI, but the most frequently used method is to divide.