At its most fundamental level, an activity is simply a collection of milestones (date and time) and data that pertain to ...

At its most fundamental level, an activity is simply a collection of milestones (date and time) and data that pertain to a given business process or processing pattern.  For example, an activity for tracking purchase orders might include milestones like "PO Received" or "PO Approved" and data items such as "Order Number" or "Product". Activities are typically defined by someone who is knowledgeable in the relevant business area and has expertise in visibility requirements for process management.