phonon
/ FOH-non /
When you pluck a guitar string it does not get arbitrarily louder or softer in a perfectly smooth way; in the quantum world, the energy of a vibration comes in countable lumps. A phonon is one such lump — a single packet, the smallest indivisible unit, of vibrational energy travelling through a crystal.
Atoms in a solid vibrate together in collective waves rather than each doing its own thing. Quantum mechanics tells us that each of these waves can only gain or lose energy in fixed steps, never in between. We give a convenient name, 'phonon', to one of those steps, and then we can talk about adding or removing phonons just as we talk about photons of light. A louder, hotter vibration simply has more phonons in it. The word is built to rhyme with 'photon', and the analogy is deliberate: a phonon is to sound and heat what a photon is to light.
Phonons matter because they let us treat heat, sound, and the resistance an electric current feels as a kind of particle bouncing around inside a solid, which makes hard problems tractable. But be careful: a phonon is not a real particle you could pull out and hold. It is a useful bookkeeping device — a quantum of a collective motion that only exists because the whole crystal moves together.
When a flowing electron bumps into the jiggling lattice and loses a little energy, physicists describe it as the electron 'emitting a phonon' — handing off one packet of vibration. Pile up countless such handoffs and you have explained why a wire warms up and resists the current passing through it.
Electrical resistance, told as a story of electrons trading phonons with the lattice.
Phonons are 'quasiparticles': they behave like particles inside the crystal but cannot exist on their own outside it. Remove the solid and there is nothing left to vibrate, so the phonons simply vanish.