A voxel is a volumetric, three-dimensional pixel. A single voxel is a cube-shaped particle that represents a point in 3D space. A group of voxels, arranged in a 3D grid, combine to create a 3D shape in a way similar to how pixels combine to create a 2D image.
As a method of creating three-dimensional graphics, voxels are a less-common alternative to 3D wireframes and polygons. Instead of building a 3D object out of vertices connected by lines and flat polygons, an object made of voxels consists of uniformly-shaped blocks attached to each other. Each voxel can have its own properties like color, transparency, and texture. An object made of voxels can be hollow, or it can include voxels inside it with their own properties; for example, a 3D model of an avocado made out of polygons would be a hollow avocado-shaped object, while one made of voxels would have an outer skin, an inside layer, and a pit in the middle.
While polygon-based 3D graphics are far more common in video games than voxels, games like Minecraft often use voxels for procedural world generation and destructive terrain effects. Scientific and medical imaging also frequently use voxel-based 3D rendering to represent objects using 3D cross-sections. For example, doctors can view a 3D medical imaging scan of a diseased organ from any angle, including a cross-section to see the problem areas inside.