Return on Investment (ROI) is a financial metric used to evaluate the profitability of an investment. It is calculated by dividing the net profit from an investment by the cost of the investment and is usually expressed as a percentage. In digital marketing, ROI is used to assess the effectiveness of marketing campaigns, comparing the revenue generated by a campaign to its cost. A high ROI means the investment gains compare favorably to their cost, indicating a successful campaign. ROI is a crucial metric for businesses as it helps in making informed decisions about where to allocate resources for the best return, ensuring the sustainability and growth of marketing efforts.