A three-dimensional object is any object with height, width and depth. The term "three-dimensional" is in reference to those three measurements. A solid object is another term for a three-dimensional object.
In the physical world, most objects, such as chairs, desks, trees and people, are three-dimensional. Even paper, which seems like it has only two dimensions, is actually three-dimensional, for it has a very small depth. In geometry, examples of three-dimensional objects include spheres, cylinders, cones, cubes and pyramids. These solid objects are plotted on a coordinate system with three axes: the x-axis, the y-axis and the z-axis. Two-dimensional objects, such as squares and circles, are plotted on a two axis system with only an x-axis and a y-axis.
An example of a three-dimensional object is a cube or pyramid. Both of these objects consist of multiple two-dimensional shapes. Because the shapes are laid out so that they share only one dimension with each other, the combined shape gains an extra dimension to become three-dimensional. People see the world through three dimensions of space because all visible objects are three-dimensional. The only two-dimensional objects are projections created on surfaces.