In general, a hegira is a journey from one place to another more desirable place. After Old Earth was destroyed in the Big Mistake, the term Hegira became used almost exclusively to refer to the mass exodus of humanity from its homeworld to the stars, beginning what would become the Hegemony of Man.
Post-Hegira Space Exploration
By the time of the Final Shrike Pilgrimage, humanity had explored over eight thousand worlds. A large majority of these worlds were explored very cursorily.
Of the eight thousand known worlds, less than 400 have been colonized by humanity. Of these, 176 are fully integrated members of the WorldWeb. The remaining 200 are colonial and protectorate Outback worlds. Nine are especially unique Labyrinthine Worlds.