Diffraction
Definition
Waves bending and spreading around obstacles and through narrow openings — why you can hear someone talking around a corner.
Technical definition
The spreading of waves past an aperture or obstacle, significant when the opening is comparable to the wavelength. Described by Huygens' principle: every point on a wavefront acts as a source of new wavelets.
Example
You hear a conversation from around a hallway corner because sound's ~1 m wavelength diffracts strongly around the wall. Light's ~500 nm wavelength barely diffracts at all — which is why you cannot see around the same corner.
Diffraction is not refraction. Refraction bends waves because speed changes between media; diffraction spreads waves because part of the wavefront is blocked — no medium change needed. And diffraction is not limited to light: it is why AM radio (long wavelength) reaches behind hills while your Wi-Fi (short wavelength) dies behind one wall.