Architecture	1: documentation describing the structure (components) and behaviour (functions) of a system. A detailed plan of the system to guide its implementation. Or 2: the process for describing the architecture of a system to meet given requirements and under given constraints.
System	“a collection of components, organized to accomplish a specific function or set of functions” (extract from ANSI 1471) A collection of interacting subsystems or components. An encapsulated collection of processes that transform inputs into outputs. (Output being the purpose of a man-made system.)
Software system	A system in which computer programs execute the processes.
Human activity system	A system in which people execute some or all of processes. A rich system, much more complex and loosely-defined than a software system. It relies to a greater or lesser degree on human personalities, skills, ingenuity and judgements about what to do and how to do it.
Structure	What a system is made of. A configuration of items; a collection of inter-related components or services. A structure may be a hierarchy (items arranged in a cascade of one-to-many relationships) or a network (items connected by many-to-many relationships).
Behaviour	What a system does. What external entities observe a system as doing. The processes executed or supported by a system.
Process	1: A procedure, started by an event, which terminates with an output or result. A set of steps or activities arranged under a control flow in one or more sequential paths. Or 2: An encapsulated system or subsystem - as in the “active process” on a computer. This reference model favours definition 1.
Component	A subsystem that is encapsulated behind an interface - can be replaced by any other with the same interface - is related to other subsystems by requesting or delivering services. A component can have several interfaces.
Interface	The means of connecting to a system, process or component. 1: A list of services a component can offer; a type. Sometimes implemented in a distinct façade component. Or 2: The name, inputs and outputs of a single service. Or 3: The protocols used to exchange data between components This reference model favours definition 1.
Service	1: An interface of a single process that does something in response to a request from a client. Or 2: An interface of a component (such as a web service) that may offer many services of kind 1. This reference model favours definition 1, to limit confusion between components and services.
Function	1: A component that offers one or more services. Or 2: A process that delivers a single service; which can involve many components. Or 3: A purpose or goal of a component or process. This reference model eschews the term function except within the term business function.
Life cycle	A process from birth to death. E.g. the life of: · a system from conception through deployment and use to removal · a project from conception through development to delivery · an entity (see Data Lifecycle).