die Aktivität
activity
The state of being active; an action or process in which one is physically or mentally active.
Regelmäßige körperliche Aktivität ist gut für die Gesundheit.
Regular physical activity is good for your health.

activity
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activity
The state of being active; an action or process in which one is physically or mentally active.
Regelmäßige körperliche Aktivität ist gut für die Gesundheit.
Regular physical activity is good for your health.

The word is derived from the adjective 'aktiv' (active). The ending '-ität' is typical of Latin loanwords and forms an abstract noun that describes a state or quality.
Die Universität fördert die internationale Aktivität ihrer Studierenden.
The university promotes the international activity of its students.
In the singular, 'Aktivität' (activity) usually describes the general state of being active. In the plural ('Aktivitäten'), it almost always refers to specific individual undertakings or hobbies.
Geistige Aktivität hält das Gehirn fit.
Mental activity keeps the brain fit.
When referring to undertakings, 'Aktivitäten' (activities) is often combined with a descriptive adjective. Common examples are 'sportliche Aktivitäten' (sporting activities), 'kulturelle Aktivitäten' (cultural activities), or 'Freizeitaktivitäten' (leisure activities).
Das Hotel bietet seinen Gästen viele Freizeitaktivitäten an.
The hotel offers its guests many leisure activities.
This meaning of 'Aktivität' (activity) is a technical term from physics and chemistry. You can recognize it because the context is scientific and units of measurement such as becquerels or moles are often mentioned.
Im Labor wurde die radioaktive Aktivität des Materials gemessen.
The radioactive activity of the material was measured in the laboratory.
movement
'die Bewegung' and 'die Aktivität' can both mean that someone is not passive. 'Bewegung' fits especially when it is about the body: walking, running, doing sports, or moving around. 'Aktivität' is more general. It can be physical, but also mental or organizational, for example a course, a game, or a task. Learners may confuse the words because sports are an activity and also involve movement. As a practical rule: for health and the body, people often say 'Bewegung'; for individual things to do or program items, 'Aktivität' is more common.

Für den Ausflug hat die Gruppe mehrere Aktivitäten geplant.
The group has planned several activities for the outing.
Nach vielen Stunden am Schreibtisch brauche ich etwas Bewegung.
After many hours at my desk, I need a bit of exercise.