generation

A 'generation' in the Standard Model is one complete set of matter particles that repeats, like one full size in a range of identical products. Nature did not stop at the minimum needed to build atoms; it made the same little family of particles two more times over, just heavier. A generation is one such family.

Each generation contains exactly four particles: an up-type quark, a down-type quark, a charged lepton, and a neutrino. The first generation is (up quark, down quark, electron, electron neutrino). The second is (charm, strange, muon, muon neutrino). The third is (top, bottom, tau, tau neutrino). Within each generation the electric charges line up identically — only the masses climb as you move to higher generations. So a generation is the unit in which the particle table repeats, and 'which generation' is one of the two main things, along with flavor, that distinguishes one matter particle from another.

Why there are exactly three generations is a genuine mystery — the model accommodates three but does not say why not two, or seventeen. Measurements of how the Z boson decays strongly suggest there are only three light neutrino types, which makes three generations look like a real fact about nature rather than an accident of how far we have looked. The relationships and tiny mixings between generations are also central to the matter-antimatter puzzle.