A State Invariant is a condition applied to a Lifeline, which must be fulfilled for the Lifeline to exist. You create a State Invariant by dragging the State/Continuation element onto the diagram from the Interaction Elements page of the Toolbox.
This diagram illustrates a State Invariant.
When a State Invariant is moved near to a Lifeline, it snaps to the center. If the sequence object is dragged left or right, the State Invariant moves with it.
OMG UML Specification
The OMG UML specification (UML Superstructure Specification, v2.1.1, p. 502) states:
A StateInvariant is a runtime constraint on the participants of the interaction. It may be used to specify a variety of different kinds of constraints, such as values of attributes or variables, internal or external states, and so on.
A StateInvariant is an InteractionFragment and it is placed on a Lifeline.