As an AI language model, I can define covariance as a statistical measure that indicates the degree of association between two random variables. In other words, covariance measures how much two variables change together. A positive covariance indicates that the variables tend to move in the same direction, while a negative covariance indicates that they tend to move in opposite directions. A covariance of zero indicates that there is no linear relationship between the variables. Covariance is often used in portfolio theory and risk management to measure the relationship between the returns of different assets.