Infection, nosocomial: An infection acquired in a hospital. Specifically an infection that was not present or incubating prior to the patient being admitted to the hospital, but occurred after admittance to the hospital.
After antibiotics came into common usage in the second half of the 20th century, bacteria resistant to antibiotics were selected for, especially in hospitals. Nosocomial infections proliferated that were (and are) resistant to antibiotics.
"Nosocomial" comes from two Greek words -- "nosus" meaning "disease" + "komeion" meaning "to take care of" -- disease contracted by a patient while under medical care.