From left to right, the square, the cube, and the tesseract. The square is a 2-dimensional object, the cube is a 3-dimensional object, and the tesseract is a 4-dimensional object. A 1-dimensional object is just a line. A projection of the cube is given since it is viewed on a two-dimensional screen. The same applies to the tesseract, which additionally can only be shown as a projection even in three-dimensional space.

A diagram of the first four spatial dimensions.

Dimensions are the way we see, measure and experience our world, by using up and down, right to left, back to front, hot and cold, how heavy and how long, as well as more advanced concepts from mathematics and physics. One way to define a dimension is to look at the degrees of freedom, or the way an object can move in a specific space. There are different concepts or ways where the term dimension is used, and there are also different definitions. There is no definition that can satisfy all concepts.