Full Definition of NEUTRINO. : an uncharged elementary particle that is believed to have a very small mass, that has any of three forms, and that interacts only rarely with other particles.— neu·tri·no·less \-ləs\ adjective.See neutrino defined for English-language learners. an uncharged elementary particle that is believed to have a very small mass, that has any of three forms, and that interacts only rarely with other particles. — neu·tri·no·less \-ləs\ adjective. See neutrino defined for English-language learners.