Architecting and designing : UML 2.0 Lite : UML models : UML Use Case diagram : Use Case Association
  
Use Case Association
The Use Case Association describes the relationship between an actor and a Use Case. Such an association states that an instance of the Use Case and a user playing one of the roles of the actor communicate. It is drawn as a solid line between the actor and the Use Case. It may have adornments such as multiplicity.
Multiplicity
To show the number of instances related through the Use Case Association line, specify its multiplicity.
See also
UML Use Case diagram