How do you define it? It's easier with Europeans and indigenous people in someplace like Australia. In Europe, many migrated in much more distant history, but eventually everyone there has ancestors from someplace else.

Perhaps it means someone who was there before another group arrives, meaning that the definition is relative, it means 'earlier'. There's also an implication that the latter group has transformed the place somehow.

A similar word in aboriginal. Some meanings include "present from the beginning ... inhabiting or existing in a land from earliest times"; "Inhabiting or occupying a country before the arrival of European colonists"; "the earliest known inhabitants of a land"; etc.