
>>Time is not THE fourth dimension, but just yet another dimension that is commonly unified with the spatial dimension for convenience in physics.<<
Indeed, time is not the 4th dimension, the 4th dimension is external space  it includes four things: width, length, volume, time. Commonly it is just known as spacetime.
A 2D can exist in a 3D world. The 2D would consist of only width and length  there is no observable volume. Think of when you watch a cartoon, or look a line you've drawn on a piece of paper.

In mathematics, I believe the answer to the question can an n dimensional object exist in an m dimensional world is yes. The interesting thing is what the "intersection" looks like. The question has no real meaning outside of math and physics as all real objects are 3D.
As to whether time is the "4th" dimension, yes, if your doing physics within the frameword of relativity. However, this is somewhat contrived as a method of defining simultaneity. Light travels at a constant velocity in free space so one can measure time as distance. Mapping the result into a 4 manifold (4 space) allows geometric solutions of physics problems. One example is the reduction of the 64 terms of mathmatical curvature in 4 space into only 16 terms in the Einstein curvature tensor via a geometric identity. (A 4 x4 x4 matrix of curvature terms collapses to a 4 x 4 matrix.)
Personally, I'm not entirely convinced that relativity is an actual physical model of the universe, but untill something better comes along it's really the only game in town.

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that is complete bullshingles.
If the object dissapears when one looks at it from a "90degree angle" the object does not exist.
and how could you turn the object to look at it? it is 2 dimensional. Turning the object would imply that it has SOME depth. Also, by turning the object, you GIVE it depth (at least in your reference point).
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When a two deminsional object is rotated it does not disappear, but would become a one deminsional obect fro the perspective of the observer.
Experiment: Take something thin and flat that has length and width (like paper), rotate it until all you see is the edge (so that you can not see either side) all you will see is a line which is a one deminsional object.

Maybe, but don't forget the fifth dimension
Let the sun shine in...

While we're at it. There's a sixth sense. It means you can see dead people.