Diffraction, in physics, refers to the change in an object's velocity as a consequence of contact (collision) with a surface or the influence of a field.

Difraction is the change in direction of wave propagation due to a change in its transmission medium.

Diffraction is the spreading out of waves after impacting on a corner / edge or travelling through a small aperture (two edges a small distance apart).

Diffraction is the change in direction of a wavefront at an interface between two different media so that the wavefront returns into the medium from which it originated. 

Diffraction is due to the particle-like nature of waves.