A sequence diagram is a type of interaction diagram used in software engineering to visualize the interactions between objects in a system and the order in which they occur. It is a time-focused diagram that shows the sequence of messages exchanged between objects. Sequence diagrams are typically associated with use case realizations in the 4+1 architectural view model of the system under development.

Some key elements of a sequence diagram include:

• Lifelines: These are parallel vertical lines that represent different processes or objects that live simultaneously.

• Messages: These are horizontal arrows that represent the messages exchanged between objects, in the order in which they occur.

• External actors: These are actors outside the system that generate events.

• Return values: These are values returned by messages.

Sequence diagrams are useful for documenting and understanding requirements for new and existing systems. They can be used in multiple stages of the development process to map out the timing of interactions, establish system requirements, and consider how individual aspects of the system contribute to the overall end goal. They can also be used to model and analyze specific use cases, and to solve interaction problems that arise.

UML has introduced significant improvements to the capabilities of sequence diagrams, such as interaction fragments which allow for more complex interactions to be modeled.