A "16-cell" is a four-dimensional geometric shape, also known as a hypercube or tesseract. It is the four-dimensional analog of a cube, just as a cube is the three-dimensional analog of a square. A 16-cell consists of 16 tetrahedral cells, which are the simplest three-dimensional shapes. In mathematical terms, the 16-cell can be represented in a coordinate system with four dimensions. Each vertex of the 16-cell can be described using four coordinates, and it has a total of 8 vertices. This shape is part of a broader study in geometry and topology, which includes other shapes like the cube and hypercube.