In the case of the triangle, The triangle is a concept. There are no triangles in the observable physical universe. A triangle is a 2D object. 2D cannot exist in a 3D world. If there is a "Triangle" that doesn't have its internal angles adding up to 180 degrees, then it's not a 2D object.

Where does that chain of definition end?

It doesn't end. It's not even just linear progression. It'll be both a web and something that's like a web. Word's should be used like the way a person use a raft to cross a river. You use it and you leave it. It's like cryptocurrency. The value lies in the function it serve. No need to cling to words beyond the use of communication.

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