Horizontal means "flat" or "having no slope" or "with a slope of zero" or the like. It's just what you think it is. It has "across" to it, but no "up" or "down" aspect. A horizontal line is one that follows the horizon, or is "straight across" from one side to the other. The "no slope" or "zero slope" speaks to the mathematical definition.
For kids: it is not up and down, it is side to side.So it is going across or sideways.
reflected horizontally
show me a horizontal line
Horizontally
horizontally
Cells that run horizontally
Stems which grow horizontally are called stolons. Stems which grow horizontally are called stolons.
When air is moving horizontally, that is known as wind.
It was such a beautiful day, I decided to spend it horizontally on the beach
Horizontally
both vertically and horizontally
vertically and horizontally. Vertically, they get taller as new growth sprouts from the top every year. Horizontally, they get thicker and branches get longer.
Horizontally
An image shot horizontally is called a landscape photograph.
left to right --------------------------------