**Covariance: **Covariance is a statistical measure that helps you understand how two different variables move together. It's used to gauge the linear relationship between these variables. A positive covariance means the variables move in the same direction, while a negative covariance indicates they move in opposite directions. If the covariance is zero, it suggests there is no linear relationship between the variables. This concept is widely used in risk management, portfolio theory and A/B testing to understand the impact of changes on different variables.