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 Unified Modeling Language specification, (v2.5.1, p.572) states:
In an interaction diagram a Lifeline describes the time-line for a process, where time increases down the page. The distance between two events on a time-line does not represent any literal measurement of time, only that non-zero time has passed.
Events on the same time-line are ordered linearly down the page, except where they occur within a parallel combined fragment, or along a lifeline within a “coregion”. (...) Within a parallel combined fragment or a coregion, events are not locally ordered unless that is directly imposed by a general ordering construct. (...).
The order of OccurrenceSpecifications along a Lifeline is significant denoting the order in which these OccurrenceSpecifications will occur. The absolute distances between the OccurrenceSpecifications on the Lifeline are, however, irrelevant for the semantics.