Dimensions - 1D-2D-3D-4D-......-nD

1D-2D-3D-.............-nD:

One dimension, say a line with LENGTH,l

This is 1D as there is only one dimension - length

If we want two dimensional, we need to have another dimension which is perpendicular to the previous dimension i.e., making 90 degrees with length, l.

So, draw a line at one end of length that is perpendicular. So, if our previous line is horizontal, now we get a vertical line, let it be breadth,b.

So, this is 2D - LENGTH and BREADTH.

Examples of two dimensional figures are SQUARE, RECTANGLE, TRIANGLE, CIRCLE, etc,.

Now 3D means one more dimension to be added to 2D and it should be perpendicular to length as well as breadth at that same point where length and breadth are perpendicular to each other.

This is not possible to draw on a paper exactly as paper itself is a 2D object as there is only length and breadth for the paper, but for the convenience people draw a line cross to length and breadth assuming it makes 90 degrees with both of them and we call it HEIGHT. If you observe a cuboid, at any point i.e., vertex, we can observe that length, breadth and height are perpendicular to each other. A paper has no height as it is 2D body - only length and breadth.

Examples of three dimensional figures are CUBE, CUBOID, CONE, SPHERE, etc,.

This world is a 3D world. Sun, moon, earth and many other bodies are all 3D figures, we can say they are spheres as they look like balls approximately, if not exactly. 

                   Now, if we want 4D, we need to have another dimension which must be perpendicular to length, breadth and height at the same point which is impossible to imagine for us, it is something which is beyond our imagination as we human beings are 3D bodies on this 3D planet. And we know upto 3D only. We do not know more that that. If one knows 4D, then such one will automatically know 1D, 2D and 3D also but may not know 5D, 6D, etc,.