meson vs pion

meson and pion both are nouns.

Word	Noun	Adjective	Verb	Adverb
meson	Yes	No	No	No
pion	Yes	No	No	No

As nouns, pion is a hyponym of meson; that is, pion is a word with a more specific, narrower meaning than meson:

meson: an elementary particle responsible for the forces in the atomic nucleus; a hadron with a baryon number of 0
pion: a meson involved in holding the nucleus together; produced as the result of high-energy particle collision

Other hyponyms of meson include b-meson, J particle, psi particle, k-meson, K particle, kaon, kappa-meson, pi-meson.

meson (noun)	pion (noun)
an elementary particle responsible for the forces in the atomic nucleus; a hadron with a baryon number of 0	a meson involved in holding the nucleus together; produced as the result of high-energy particle collision