A neat line of things side by side is called a stack or row.
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As you can see, this square is made up of two triangles. The line drawn down the middle, from corner to corner, is the line you are asking about, that creates the "two things" or the two triangles on either side of it.
The broken line on your side means you can cross over while the people in the other lane can't.
A straight line passing from side to side going through the middle of a circle or sphere is a diameter.
side
A half line? A ray?