3.5mm headphone jack - definition
The 3.5mm headphone jack is an industry standard audio plug. Most commonly used for connecting a pair of stereo headphones to your smartphone or 'piping' your audio from your phone to an external amp either in your home or your car.
Dependent on the number of connector rings, headphones can also include a microphone.
The current trend is for high-end smartphone flagships to exclude the 3.5mm jack, with headphone support being provided via Bluetooth.
The exclusion of the 3.5mm jack is polarising with smartphone manufactures claiming that removing the jack saves space internally, allows for thinner phones while eliminating a potential ingress point for water.