1. Collective name for four pseudo- scalar mesons, having masses of about 495 MeV (megaelectronvolts) and decaying via weak interactions: K⁺, K ^-, K_S⁰, and K_L⁰; they consist of two isotopic spin doublets, the (K⁺, K⁰) doublet and its antiparticle doublet (K ^-, K̄⁰), having hypercharge or strangeness of +1 and -1 respectively, where K⁰ and K̄⁰ are certain combinations of K_L⁰ and K_S⁰ states. Also known as kaon.

2. Collective name for any meson resonance belonging to an isotopic doublet with hypercharge +1 or -1, denoted K_JP(m ) or K̄_JP(m ) respectively, where m is the mass, and J and P are the spin and parity.