The term activity in HCI often means something longer lasting than a task. For example, 'writing a book' or even 'writing books' is an activity, whereas proof reading a chapter would be task; however, the boundry is not sharp.
The notion of 'activity' is central to activity theory, which has influenced its use more widely in HCI. However, the word activity is often used informally, and may be almost synonymous with 'project' or 'role'.
Used in glossary entries: activity theory, role, task