Streptococcus pneumoniae (pneumococcus): A type of bacterium that comes in pairs and is shaped like a lancet (a surgical knife with a short wide two-edged blade).
Vaccines are available against the pneumococcus.
The formal name of the bacterium is currently Streptococcus pneumoniae. (The "strepto-" means twisted and "-coccus" comes from the Greek kokkos meaning berry.) Because of its coming in pairs, it has also been called Diplococcus pneumoniae. "Pneumoniae" refers to its propensity to settle in the lungs and cause pneumonia.