pneumococcusPronunciation: /ˌnjuːmə(ʊ)ˈkɒkəs/Definition of pneumococcus noun (plural pneumococci /-ˈkɒk(s)ʌɪ, -ˈkɒk(s)iː/) a bacterium associated with pneumonia and some forms of meningitis. Streptococcus pneumoniae, a Gram-positive diplococcus Derivatives pneumococcal adjective pneumococcus in other Oxford dictionaries Definition of pneumococcus in the US English dictionary