(physics) Any of three semistable mesons, having positive, negative or neutral charge, composed of up and down quarks/antiquarks.

(nonce word) A word of disputed meaning; perhaps, abounding in marsh marigolds.

(physics, of a theory) That includes pions

plural of pion

A theoretical type of antimatter rocket which uses proton-antiproton fuel, and magnetic rocket funnels to direct the π-on exhaust. The pi meson particles are created by the matter-antimatter annihilation of protons and antiprotons.

plural of pion rocket

In a pioneering manner

(communism) A neckerchief worn by a young pioneer, typically red in colour.

plural of pioneer scarf

plural of Pioneer