I am translating a Use Case diagram step into an Activity Diagram. Does every step of my UC diagram describe an Activity ? Is there one and only one Activity in an Activity Diagram ?
An activity is an ongoing nonatomic execution within a state machine. The ovals on an activity diagram are either action states or activity states. Action states are the atomic execution of an operation. Activity states are nonatomic and can be decomposed on another activity diagram. So, no, there does not have to one and only only activity per activity diagram. And, no, every step of a use case does not necessarily correspond to an activity. They may be action states or activity states or possibly neither.