A linear relationship between two variables means that when one variable changes, the other variable changes in a fixed proportion. This can be represented mathematically as: \[ Y = a + bX \] where \( Y \) is the dependent variable, \( X \) is the independent variable, \( a \) is the intercept, and \( b \) is the slope. The slope \( b \) indicates the rate of change of \( Y \) with respect to \( X \).