In Unified Modeling Language (UML) sequence diagrams, a lifeline represents a connectable element of the enclosing classifier, including inherited properties or parameters, and describes how each instance participates in the interaction over a period of time. Each instance in an interaction is represented by a lifeline.
As the following figure illustrates, a lifeline consists of a name in a rectangle, which is called the head, on top of a vertical dashed line, which is called the stem, in an interaction frame.
The instance name and type appears in the head of the lifeline. The stem represents the instance that the lifeline represents. Messages, which are sent and received by the instance, appear on the lifeline in a chronological order.