Is a horizontal line a linear function?

Yes, a horizontal line is considered a linear function. In the form of (y = b), where (b) is a constant, it represents a relationship where the output (y) remains the same regardless of the input (x). This satisfies the definition of a linear function, which can be expressed as (y = mx + b), with a slope (m = 0). Thus, horizontal lines are a specific case of linear functions.