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 ...

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/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.thefreedictionary.com/Horizontal+line

**Horizontal line**. Also found in: Encyclopedia, Wikipedia. Related to **Horizontal line**
: Vertical line. (Descriptive Geometry & Drawing), a constructive line, either ...

www.investopedia.com/terms/h/horizontal-line.asp

**DEFINITION** of '**Horizontal Line**'. In finance and economics, a **horizontal line** is
usually the X axis in a data plot. It is a line that appears to proceed from left to
right ...

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 ...