According to the spanish royal academy is the habitant of the cities that have political rights.
In my opinion the active citizen is the person lawfully exercising rights and duties.
For example when citizens meet the coming of age and participate in choosing their leaders with the right to vote, could be reflected in the analysis of leaders, evaluation of local proyects, participation on different topics about the goverment, the neigborhood develop among others.

In conclusion an active citizen participates democratically in the develop of the territory and knows his duties and rigts as a citizen !!.