Linear Function:A linear function has the form $f(x) = mx + b$, where $m$ and $b$ are constants. Its graph is a straight line.
Horizontal Line: A horizontal line has an equation of the form $y = c$, where $c$ is a constant. This represents a constant function.
$y$-intercept: $y$-intercept is the point where the graph intersects with the y-axis. For a constant function, this occurs at $(0, c)$.