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.
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