meson vs pi-meson

meson and pi-meson both are nouns.

Word	Noun	Adjective	Verb	Adverb
meson	Yes	No	No	No
pi-meson	Yes	No	No	No

As nouns, pi-meson is a hyponym of meson; that is, pi-meson 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
pi-meson: 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, pion.

meson (noun)	pi-meson (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