Definition of "Quark" :

noun: fresh unripened cheese of a smooth texture made from pasteurized milk, a starter, and rennet

noun: (physics) hypothetical truly fundamental particle in mesons and baryons; there are supposed to be six flavors of quarks (and their antiquarks), which come in pairs; each has an electric charge of +2/3 or -1/3

"Quarks have not been observed directly but theoretical predictions based on their existence have been confirmed experimentally."