Ma'anar ƙamus na kalmar "capsid" ita ce:nounprotein harsashi da ke kewaye da kwayoyin halitta (ko dai DNA ko RNA) na kwayar cuta. Capsid wani muhimmin abu ne na kwayar cuta, domin yana kare kwayoyin halitta daga lalacewa kuma yana ba da damar kwayar cutar ta shiga cikin kwayoyin halitta kuma ta yi kwafi. Ya ƙunshi maimaita subunits sunadaran da ake kira capsomeres, wanda zai iya bambanta da siffa da girma dangane da kwayar cutar. Har ila yau, capsid ne ke da alhakin tantance siffar kwayar cutar, wanda zai iya zama ko dai helical ko icosahedral, da sauran siffofi.