Word for “direction“ where which way you're pointing to doesn't matter?

Question

Serbian language has a distinction between “smer” and “pravac”.

• “smer” is a straight line when you turn towards something. It's similar to a vector or a ray in maths. For example, “left”.
• “pravac” is a straight line that extends behind you. It's like a line in maths: there's no preferred way about which is “forward” and which is “backwards”. For example “left-right”.

We usually extends these meanings outside of “straight line”. For example, a train is going back-and-forth between towns A and B means that its “pravac” is A-B. In other words, it goes both from A to B and from B to A, but the line it follows is the same.

It doesn't mean just a “road”; it's specifically implying movements in both directions. For the same reason, it's not a “line”, although we do say “line” for buses for example -- which go back-and-forth between two destinations. But this feels more like a phrase “bus line”, and without a context “line” sounds just a regular straight line drawn with a pencil. “Way” also comes to mind, but this

How do I casually differentiate between these in English?

I head people using direction for both meanings, although with a heavy bias towards “smer” (when start and end points are different).

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I don't know how to tag this. It doesn't necessarily have to be one word, nor am I looking for translation; I've used my native language as an example to describe the difference. This is similar, but asks specifically in mathematical context.

Answer 1

What a coincidence! I had a very similar problem recently in trying to explain to someone how a polaroid (polarising filter) works. Its molecules are arranged parallel to each other and will stop the propagation of light having an E vector pointing one way along the molecules or in the opposite direction along the molecules. I wanted to say that it stops light with its E vector pointing in the same direction as the molecules, but that's not quite right, as we both understand.

The word I needed is 'alignment'. The polaroid stops light whose E vector has the same alignment as the molecules.

That solved my problem, but it's hardly casual or informal usage, and it won't be applicable in every case when you want to distinguish between a road aligned (say North–South) and a particular direction along that road. Not as versatile as smer and pravac.