An additional dimension to the traditional three: length, width, depth. It is most often associated with time, although it could very well represent other variables, such as temperature or attraction, as it is, literally, just another quality to describe something.
There are conjectures that there may be ten total dimensions, even more, or even an infinite amount. The thought concerning dimensions past 3D is generally in the realm of the theoretical.
Since we see 2D in 3D (what you actually see is a 2D image), we would see true 3D when looking from 4D.
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