Horizontal line (geometry)

A horizontal line is a straight line parallel to the horizon, with constant vertical position in a coordinate system.

Cartesian coordinate system: A coordinate system that locates points by signed distances along perpendicular axes. Its vertical axis defines the coordinate that stays fixed along a horizontal line.
Slope-intercept form: The equation form y = mx + b, where m is slope and b is the vertical intercept. Setting m to zero gives the equation of a horizontal line.
Constant function: A function that assigns the same output to every input in its domain. Its graph is a horizontal line at the function's constant value.
Vertical line: A line parallel to the vertical axis, with constant horizontal position in a coordinate system. It reverses which coordinate remains fixed and has undefined slope in Cartesian coordinates.
Slope: A measure of a line's steepness, commonly expressed as vertical change divided by horizontal change. A horizontal line has zero vertical change and therefore slope zero.
Linear equation: An equation whose graph is a straight line or, in higher dimensions, a linear set. A horizontal line is the graph of a linear equation with fixed y-coordinate.
Level set: The set of inputs at which a function takes a specified value. For a one-variable function, a horizontal line identifies inputs in a level set.
Oblique line: A line that is neither horizontal nor vertical relative to a chosen coordinate system. Its direction changes both coordinates rather than keeping vertical position constant.
Parallel lines: Distinct coplanar lines that never intersect and remain the same distance apart. Horizontal lines share a direction with the coordinate system's horizontal axis.
Y-intercept: The point where a graph intersects the vertical axis. A horizontal line's y-intercept determines its constant vertical coordinate.

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