"4D" refers to the concept of four-dimensional space, which includes three dimensions of space (length, width, height) plus an additional dimension, often considered time. In physics, this concept is essential for understanding the universe, as it allows for the description of how objects move and interact over time. In mathematics, 4D can also refer to geometric shapes that extend beyond our usual three-dimensional understanding. For example, a tesseract is a four-dimensional analog of a cube. While we cannot visualize 4D directly, we can represent it through projections or models that help us grasp its properties.