Actually, a line is 2d. If it were only 1d, you wouldn't be able to see it as it had no thickness. And Terraria is not 2.5D, other than NPC/Enemy Spawn mechanics and torch placement, the background does not factor into gameplay whatsoever.
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Xylia
Proxy
a line is 1D and i thought 2.5D games are Games with 3D models on a 2D Gameworld, or 2D Sprites which are Simulating a near 3D world... here: www.en.wikipedia.org/wiki/2.5D aaaannndd here for the Line thing: www.msnlv.com/one-dimensional-world.jpg
Azu
1 D means one dimension. A line has two dimensions, length and width. Therefore a line is 2D.
Proxy
take Axis for Example, a Line has only length, only one Axe (x), a Square has Lenght and Width (x and y) and a Cube has Length, width and Depth (x, y and z)... got it? if not: here the link again :P www.msnlv.com/one-dimensional-world.jpg