A linear function is a mathematical relationship between two variables that can be represented by a straight line on a graph. It is typically expressed in the form of an equation, such as y = mx + b, where m is the slope of the line, indicating its steepness, and b is the y-intercept, the point where the line crosses the y-axis. Linear functions are characterized by a constant rate of change, meaning that for every unit increase in x, the value of y changes by a fixed amount. This property makes linear functions useful in various fields, including economics, physics, and engineering, where they can model relationships like cost, distance, or speed.