The kind of function without an X variable.
Y = 4
is a horizontal line.
parallel
The two equations are linear.
linear b
NONE
horizontal
Linear A script is a kind of writing system, that Minoan culture used.
You don't really need to know this unless you have any drive, or desire to rise above the dumbified populations and make the world a better place through science. If you do want to do something of value, not just asking people if they want their number 1 super-sized, graphing linear equations is a step in understanding higher level mathematics - the kind of math they used to get folks on the moon, design cars, create amortization schedules, etc.
Acceleration
Of course, equations are going to be studied in arithmetic while in elementary school. A different kind of equations will be studied in algebra and calculus in high school.
Motion Perpendicular to the ground is called horizontal motion.
Linear
One with sufficiently roomy, horizontal or near-horizontal, permanently dry entrance passages.