An isometrical drawing is a nearly 3d drawing showing the object's width and depth in a complete image, from each curved plane of the orthhographic view, the viewpoint is at a 45 degree angle. From an observations point of view, isometric differs, since all longitudes are true.
An orthographic drawing is one that provides consistently measured non-perspective views from above, front and right side. Using Orthographic projection one can then develop a measured pseudo- perspective drawing:
An isometric drawing is a form of mechanical perspective drawing in which receding 90º angles are drawn at a consistent 30º. There is no convergence toward a vanishing point so it is not a true vanishing point perspective.