Quark
Definition
The tiny building blocks inside protons and neutrons — six types that combine to make most of the matter around us.
Technical definition
Elementary fermions with fractional electric charge (±⅓e, ±⅔e) that combine via the strong force: baryons (3 quarks, e.g., proton = uud) and mesons (quark-antiquark). Six flavors: up, down, charm, strange, top, bottom. Never observed in isolation (color confinement).
Example
A proton is two up quarks and one down quark (uud) — their charges (+⅔ +⅔ −⅓) add up to exactly +1.
Common confusion
Quarks cannot be isolated — pulling them apart creates new quark-antiquark pairs from the energy invested (confinement). ‘Seeing’ a quark really means detecting its jets of particles, never the quark alone.