I always wondered, why the European occupation of foreign nations is called colonialism, whereas Arabic invasion and later occupation of Spain and Portugal is not? Similarly, why do we not call the Mongolian expansion to the eastern European states as colonialism?

Just like the Europeans, Arabs too, have built and established cities, political institutions and have influenced regions that were under their control? Likewise, the Mongols have issued taxes, exploited natural resources and sent settlers to occupy the areas which were under their rule.

What exactly defines colonialism, and why do we not call Asian civilizations of the past as colonial empires?