Terms and definitions（术语和定义）
Event: generic term used to describe an occurrence of some significance within a system or system component.
Condition : generic term that is an extension to an Event.
Alarm ： type of Event associated with a state condition that typically requires acknowledgement.
Notification: generic term for data that announces the detection of an Event or of a changed Attribute value;Notifications are sent in NotificationMessages.
EventNotifier: special Attribute of a Node that signifies that a Client may subscribe to that particular Node to receive Notifications of Event occurrences.
MonitoredItem: Client-defined entity in the Server used to monitor Attributes or EventNotifiers for new values or Event occurrences and that generates Notifications for them.
Object: Node that represents a physical or abstract element of a system.Objects are modelled using the OPC UA Object Model. Systems, subsystems and devices are examples of Objects. An Object may be defined as an instance of an ObjectType.
Object Instance: synonym for Object. Not all Objects are defined by ObjectTypes.
ObjectType: Node that represents the type definition for an Object.
Complex Data: data that is composed of elements of more than one primitive data type, such as a structure.
Attribute ：primitive characteristic of a Node. All Attributes are defined by OPC UA, and may not be defined by Clients or Servers. Attributes are the only elements in the AddressSpace permitted to have data values.是AddressSpace中唯一允许具有数据值的元素。
Variable: Node that contains a value.
Method: callable software function that is a component of an Object.
Service: Client-callable operation in an OPC UA Server. Services are defined in Part 4. A Service is similar to a method call in a programming language or an operation in a Web services WSDL contract.