Encephalopathy /ɛnˌsɛfəˈlɒpəθi/ means disorder or disease of the brain. In modern usage, encephalopathy does not refer to a single disease, but rather to a syndrome of global brain dysfunction; this syndrome can be caused by many different illnesses.

In some contexts it refers to permanent (or degenerative) brain injury, and in others it is reversible. It can be due to direct injury to the brain, or illness remote from the brain. In medical terms it can refer to a wide variety of brain disorders with very different etiologies, prognoses and implications. For example, prion diseases, all of which cause transmissible spongiform encephalopathies, are invariably fatal, but other encephalopathies are reversible and can be caused by nutritional deficiencies, toxins, and several other causes.

There are many types of encephalopathy. Some examples include:

The hallmark of encephalopathy is an altered mental state. Depending on the type and severity of encephalopathy, common neurological symptoms are loss of cognitive function, subtle personality changes, inability to concentrate, lethargy, and depressed consciousness. Other neurological signs may include myoclonus (involuntary twitching of a muscle or group of muscles), asterixis (abrupt loss of muscle tone, quickly restored), nystagmus (rapid, involuntary eye movement), tremor, seizures, jactitation (restless picking at things characteristic of severe infection), and respiratory abnormalities such as Cheyne-Stokes respiration (cyclic waxing and waning of tidal volume), apneustic respirations and post-hypercapnic apnea.