"Diplo-" refers to a bacterial arrangement consisting of two bacteria. One of the most common examples is diplococci, which refers to a pair of coccus bacterium.
The other types of bacterial arrangements are staphylo, strepto, sarcina and tetrad. These arrangements apply to the shapes of the arrangement rather than the numbers of bacteria. Staphylo, for example, refers to a cluster. Hence, staphylococcus is an arrangement in which cocci bacteria are grouped into irregular clusters.
Arrangement patterns apply to the bacterial colony as well as to individual bacterium. This branch of microbiology, known as morphology, classifies bacterial colonies as round, irregular, curled or filamentous.