Diffraction's normally taken to refer to various phenomena which occur when a wave encounters an obstacle. It's described as the apparent bending of waves around small obstacles and the spreading out of waves past small openings. Very similar effects are observed when there's an alteration in the properties of the medium in which the wave's travelling, for example a variation in refractive index for light waves or in acoustic impedance for sound waves and these can also be referred to as diffra… (More on Diffraction)