What is the meaning of Heathendom?

The state of being heathen.

From one religion's or creed's perspective, the rest of the world that does not follow that creed or religion.

Specifically, the non-Christian world; territories where Christianity is not the dominant religion.

The collection of people who follow Heathenry, a modern pagan faith inspired by the pre-Abrahamic religions of Germanic tribes, Anglo-Saxons and Norse peoples.

Source: wiktionary.org