en.wikipedia.org/wiki/Horizontal_line_test

In mathematics, the **horizontal line** test is a test used to determine whether a
function is injective ... (because they lie on the line y = c {\displaystyle y=c} y=c )
but different x values, which by **definition means** the function cannot be injective.

www.mathopenref.com/horizontal.html

In geometry, a **horizontal line** is one which runs from left to right across the page.
It comes from the word 'horizon', in the sense that **horizontal lines** are parallel to ...

www.mathopenref.com/coordhorizontal.html

Definiton and equation for a **horizontal line** in coordinate geometry.

virtualnerd.com/algebra-1/linear-equation-analysis/intercept/horizontal-vertical-lines/horizontal-line-definition

It's a **horizontal line**! And just like the horizon, **horizontal lines** go straight left and
right. ... **definition**; horizontal; graph; graphical; equation; horizontal; flat ...

study.com/academy/lesson/what-are-horizontal-lines-definition-equations-quiz.html

In this lesson, we will learn about **horizontal lines**. What properties are unique to
**horizontal lines**? How do you recognize the graph or equation...

www.moomoomath.com/Horizontal-Vertical-Lines.html

What are the properties of a **horizontal line**, and a vertical line. Complete
information about a **horizontal line** and a vertical line.

study.com/academy/lesson/horizontal-line-test-definition-lesson-quiz.html

The **horizontal line** test is a test used to determine if a function is a one-to-one
function. This lesson will **define** how to perform the horizontal...

www.khanacademy.org/math/algebra/two-var-linear-equations/hor-and-ver-lines-alg1/v/examples-of-slopes-and-equations-of-horizontal-and-vertical-lines

The slope of a **horizontal line** = 0 because there is no change in Y, there is only a
..... **What is** the equation of the vertical line through negative five comma ...

www.mathsisfun.com/definitions/horizontal.html

**Definition** of. **Horizontal**. **Horizontal**. Going side-to-side, like the horizon. Parallel
to the horizon. (Up-down is called vertical). See: Vertical ...