Architecting and designing : UML 2.0 Lite : UML models : UML activity diagram : Initial action state
  
Initial action state
You may model the initial state that the system is in when the sequential flow represented by the Activity diagram begins. The initial state is represented by a small, filled-in circle (or large bullet). The definition of an Initial Action State is the same as that of an Action State.
See also
UML activity diagram