The "Third Dimension" refers to the spatial dimension that adds depth to the two dimensions of height and width. In simple terms, it allows objects to have volume and occupy space, making them appear more realistic. This dimension is essential in our everyday experience, as we perceive the world around us in three dimensions. In mathematics and physics, the third dimension is often represented using a three-dimensional coordinate system, typically denoted as x, y, z. This system helps in visualizing and calculating the position of points, lines, and shapes in space, which is crucial in fields like architecture, engineering, and computer graphics.