An activity is a type of resource which you can add to Wikiversity.
Activities are a broad range of things which usually motivate a learner to do more than just edit the wiki. Wikiversity contributors have created (among other things): challenges, experiments, games, labs, learning activities, quizzes and simulations. Activities are normally included into larger projects such as courses and lessons, although there are also excellent resources which are simply collections of activities.
You should feel free to create new kinds of activity not already listed here. Don't forget to tell everyone about your new kind of activity.
Other meanings of "activity"
Some contributors use the word "activity" to refer to formal academic undertakings such as conferences, workshops and expeditions. See Help:Event.