The Collaborative International Dictionary of English v.0.48:

Diplococcus \Dip`lo*coc"cus\, n.; pl. Diplococci. [NL., fr.
   Gr. diplo`os twofold + ko`kkos grain, seed.] (Biol.)
   A form of micrococcus in which cocci are united in a binary
   manner. See Micrococcus.
   [1913 Webster]

WordNet (r) 3.0 (2006):

diplococcus
    n 1: Gram-positive bacteria usually occurring in pairs