State Invariant   A state invariant is an interaction fragment that places a constraint on the state of the lifeline on which it is located. The lifeline represents a set of instances and the constraint applies to these instances. It is evaluated at runtime and if the state invariant evaluates to false the message is considered to be invalid.

Explanation

A state invariant is a constraint that specifies that the condition (state) of the instance represented by the lifeline remains constant (invariant). As events occur and messages target the lifeline the instance of the classifier can change its state in response to these messages. A state invariant is a way of specifying constraints on these changes. During the runtime of the subject represented by the interaction and every time a message occurs that targets the lifeline the state invariant is checked, if it evaluates to true the message is considered to be valid if it evaluates to false the message is considered invalid.