Airspace of defined dimensions identified by an area on the surface of the earth wherein activities must be confined because of their nature and/or wherein limitations may be imposed upon aircraft operations that are not a part of those activities.
Source: FAA JO 7110.65 ATC
Special use airspace (sometimes referred to as "special use area(s)") is a general term that encompasses various types of segregated airspace where the activities performed within may impact air operations. Examples of such airspace types are:
Prohibited area - flights in this airspace are prohibited, usually in order to protect some installation on the ground (e.g. a military base, a nuclear plant, etc.).
Restricted area - flights in this airspace are allowed under certain conditions (e.g. if the aircraft is below certain MTOM or is performing a special mission, such as SAR).
Danger area - the activities performed in this airspace are hazardous to flights (e.g. launching of anti-hail rockets, artillery firing, etc.).
Temporary segregated area (TSA) - the airspace is reserved for a single user performing activities that may be hazardous to other traffic (e.g. airforce training, parachute jumping, etc.)