A Lifeline is an individual participant in an interaction (that is, Lifelines cannot have multiplicity). A Lifeline represents a distinct connectable element. To specify that representation within Enterprise Architect, right-click on the Lifeline and select the 'Advanced | Instance Classifier' option. The 'Select <Item>' dialog displays which you use to locate the required project classifiers.
Lifelines are available in Sequence diagrams. There are different Lifeline elements for Timing diagrams (State Lifeline and Value Lifeline); however, although the representation differs between the two diagram types, the meaning of the Lifeline is the same.
OMG UML Specification:
The OMG UML specification (UML Superstructure Specification, v2.1.1, p.489) states:
A lifeline represents an individual participant in the Interaction. While Parts and StructuralFeatures may have multiplicity greater than 1, Lifelines represent only one interacting entity.
Lifeline is a specialization of NamedElement.
If the referenced ConnectableElement is multivalued (i.e. has a multiplicity > 1), then the Lifeline may have an expression (the 'selector') that specifies which particular part is represented by this Lifeline. If the selector is omitted this means that an arbitrary representative of the multivalued ConnectableElement is chosen.