Any of a group of four mesons, one positively charged and one negatively charged, having a mass 966 times that of an electron, and two electrically neutral, having a mass 974 times that of an electron. The two charged kaons have the same mean lifetime, about 1.24 × 10^-10 second; the two neutral kaons have lifetimes of about 9 × 10^-11 second and 5 × 10^-8 second. Also called K meson.