Nosology (from Ancient Greek νόσος (nosos) 'disease', and -λογία (-logia) 'study of') is the branch of medical science that deals with the classification of diseases. Fully classifying a medical condition requires knowing its cause (and that there is only one cause), the effects it has on the body, the symptoms that are produced, and other factors. For example, influenza is classified as an infectious disease because it is caused by a virus, and it is classified as a respiratory infection because the virus infects and damages certain tissues in the respiratory tract. The more that is known about the disease, the more ways the disease can be classified nosologically.

Nosography is a description whose primary purpose is enabling a diagnostic label to be put on the situation. As such, a nosographical entity need not have a single cause. For example, inability to speak due to advanced dementia and an inability to speak due to a stroke could be nosologically different but nosographically the same.