Diffraction is the spreading of waves through a gap or around an obstacle

Interference & Diffraction — AQA A-Level Physics

Key Definition

Diffraction — The spreading out of waves after they pass through a narrow gap or around an obstruction.

Maximum diffraction occurs when the gap width is approximately equal to the wavelengthThe minimum distance between two points on a wave that are in phase (e.g. crest to crest). Measured in metres (m)..
If the gap is much larger than the wavelengthThe minimum distance between two points on a wave that are in phase (e.g. crest to crest). Measured in metres (m)., negligible diffraction occurs.
If the gap is much smaller than the wavelengthThe minimum distance between two points on a wave that are in phase (e.g. crest to crest). Measured in metres (m)., no diffraction occurs -- the wave is blocked.
When a wave diffracts, only the amplitudeThe maximum displacement of a point on a wave from its equilibrium (rest) position. Measured in metres (m). changes (decreases). The wavelength stays the same.
Examples: radio waves diffracting around buildings, water waves diffracting through a harbour entrance, X-rays diffracting through crystal lattice gaps.

Examiner Tips and Tricks

When drawing diffracted waves, keep the wavelength (distance between wavefronts) constant.
Only the amplitudeThe maximum displacement of a point on a wave from its equilibrium (rest) position. Measured in metres (m). decreases.
If you change the wavelength in your drawing, you will lose marks.