x-coordinates :)
Horizontal
When a line segment connecting two points is horizontal the length of the segment can be found by finding the absolute value of the difference in x-coordinates of the two points.
Vertical.
The absolute difference in the vertical direction is zero but the absolute difference in the horizontal direction will be the horizontal distance - which is the distance between the points.
y
absolute value verctor
vertical
Add the x-coordinates of the points and take the absolute value
The answer depends on the context: If you have a distance vector of magnitude V, that is inclined at an angle q to the horizontal, then the horizontal distance is V*cos(q).
Lines of longitude are vertical but they measure horizontal distance(In degrees,not kilometers or miles)between Greenwich Mean Time(GMT) and you so the lines are vertical,not horizontal. However,longitude measures horizontal distance,not vertical distance.
A good definition might be the measurement of the longest horizontal distance of an object (x). Width would be the shorter horizontal distance (y)
It is called the displacement in the horizontal direction.