Return on investment (ROI) is a financial metric that is used to measure the efficiency of an investment or to compare the effectiveness of different investments. It is calculated by dividing the net profit that an investment generates by the cost of the investment, and expressing the result as a percentage.

For example, if an investment costs $100 and generates a net profit of $20, the ROI would be 20% ($20 / $100). The higher the ROI, the more efficient the investment is considered to be.

ROI is commonly used to evaluate the performance of investments in business, finance, and marketing. In the context of marketing, ROI is often used to measure the effectiveness of ad campaigns and to compare the results of different marketing tactics. For example, an advertiser might compare the ROI of a display ad campaign to the ROI of a social media campaign to see which is more effective at driving conversions.