The term "isometric" comes from the Greek word for "equal measure". It implies that in a projection, the scale along each axis of the projection is the same.
In isometric views, all lines on each axis are parallel to each other, and the lines do not converge. In computer graphics it gives a 3D model representation without the distortion caused by a true perspective view.
Orthographic is derived from the Greek words ORTHOS and GRAPHOS, which mean straight drawn. Any 3D object/model can be well defined through 2D drawings. Depending upon the complicity of the shape one, two or three drawings may be required to define any object. These drawings are projections of the object and each projection/view is at right angle to the other.