A lot more could be said, but you might find my discussion of the Indigeneity of the Saami versus the Scandinavian Norse in this thread interesting.

The term "indigenous" as a qualifier for legal rights

You asked about the legal ramifications.

This is of course a history sub, not a law sub. Mods can evaluate whether a comment on how the term is used in law is within the rules. I think the way the terms are used legally is of interest to their use in history, the legal use in some cases deviates from some of the usage so far in this thread. But I'm ok with this being deleted as well.

The term indigenous is legally relevant, but it's worth noting that most of the rights of indigenous peoples are either general minority rights, or general individual rights.

But there are occasions where a definition of indigenous people (or "people") is the qualifier of certain rights. Indigenous people specifically is used in the ILO-169, and in the non-binding UN Declaration on the Rights of Indigenous Peoples, covering a wide array of rights both to autonomy, property and more.

The two "main" definitions in law

You can generalize the definitions of indigenous peoples into two groups: The Pre-Colombian definition and what I will call the functional definition.

The "Pre-Colombian" definition is illustrated by the ILO-169, which defines indigenous peoples as (art. 1(b)) "peoples in independent countries who are regarded as indigenous on account of their descent from the populations which inhabited the country, or a geographical region to which the country belongs, at the time of conquest or colonisation or the establishment of present state boundaries and who, irrespective of their legal status, retain some or all of their own social, economic, cultural and political institutions."

This definition is well suited for the contexts of colonisation and the rights of colonised peoples, at least versus their colonisers. It is, however, quite unsuited in contexts of peoples who have a clearly distinct usage or relationship with a territory, but one that was established after colonisation. It is also less useful in the context of conflict between peoples whose usage of the territory both predate colonialism done by a third people.

Because of this, the ILO-169 also includes a concept of "Tribal peoples", which have the same rights as "indigenous peoples". They are defined as (art. 1(a)): "Tribal peoples in independent countries whose social, cultural and economic conditions distinguish them from other sections of the national community, and whose status is regulated wholly or partially by their own customs or traditions or by special laws or regulations"

This definition of tribal is more "functional", in the sense that it sets up criteria meant to define whether a groups is in need of protection, rather than requiring a certain history. That is why I call it a functional definition. In the case law of the American or African courts of human rights, the term "indigenous" is used largely to cover both these definitions.

Example cases

I will use two cases to illustrate why there are two definitions, and why the wider "functional" definition is sometimes needed.

First is the Endorois case from the African Commission on Human Rights. The case was about the Endorois people, who had been evicted by the Kenyan government from their traditional areas. The commission would ordinarily not have needed to define them as an indigenous people, as the case regarded violation of "standard" rights like rights to property, culture, development etc. But because the government disputed that they were a special group that could have any rights (rather, the government considered them a part of another larger group), the commission had to deal with how to define them.

The Commission noted that a lot of the Charters rights depended on being a separate "people" (indigenous peoples included), and defined indigenous peoples by four criteria "the occupation and use of a specific territory; the voluntary perpetuation of cultural distinctiveness; self-identification as a distinct collectivity, as well as recognition by other groups; an experience of subjugation, marginalisation, dispossession, exclusion or discrimination. The Working Group also demarcated some of the shared characteristics of African indigenous groups: … first and foremost (but not exclusively) different groups of hunter-gatherers or former hunter-gatherers and certain groups of pastoralists…… A key characteristic for most of them is that the survival of their particular way of life depends on access and rights to their traditional land and the natural resources thereon".

We see that this definition used by the Commission is similar to the functional definition used in ILO-169 to define "tribal" peoples.

The second case is a set of two cases, Moiwana v Suriname and Saramaka v. Suriname. Both cases are about descendants of slaves, taken from Africa to Suriname, who fled and established themselves as distinct communities in Suriname in the 1700s and onwards. These groups were clearly not an indigenous people by the "pre-colombian" definition (being forced to the area by colonisation). But the court said that, in the question of their property rights: "this Court has held that, in the case of indigenous communities who have occupied their ancestral lands in accordance with customary practices –yet who lack real title to the property – mere possession of the land should suffice to obtain official recognition of their communal ownership

(...) The Moiwana community members are not indigenous to the region; according to the proven facts, Moiwana Village was settled by N’djuka clans late in the 19th Century

(...) the Moiwana community members, a N’djuka tribal people, possess an “all-encompassing relationship” to their traditional lands, and their concept of ownership regarding that territory is not centered on the individual, but rather on the community as a whole. Thus, this Court’s holding with regard to indigenous communities and their communal rights to property under Article 21 of the Convention must also apply to the tribal Moiwana community members: their traditional occupancy of Moiwana Village and its surrounding lands – which has been recognized and respected by neighboring N’djuka clans and indigenous communities over the years (supra paragraph 86(4)) – should suffice to obtain State recognition of their ownership."

Concluding thoughts

So, legally, indigenous rights are reserved for groups that, as the African Commission states: (1) occupies a specific territory; (2) are culturally distinct; (3) identify as, and are recognised as, a distinct group, and; (4) have an experience of marginalisation or similar.

Also, for groups that: (1) predate colonisation, conquest or/and modern state borders and (2) have retained their culture, institutions, way of life and more.

All peoples that fit either of those definitions can be called indigenous. Alternatively, indigenous can be used more narrowly to mean only those that qualify by the latter definition. If so, one can use the term "tribal" (or some other word, given that tribal can have negative connotations) for the other people that qualify by the first definition but not the latter.

Because law focuses more on who needs rights rather than historical accuracy of a definition, I don't consider it hugely important whether one uses "indigenous" for all of these situations, or prefers to say that both "indigenous" and "tribal" peoples are protected by indigenous rights.

This definition should also explain why we don't talk of the Icelandic people as indigenous. There is no other people to which we can compare them and say that they are distinct, marginalised, or have had to "retain" their culture, even if they are indigenous to the area in a grammatical sense. Really, they don't need indigenous rights.

Lastly it's worth mentioning that non-indigenous peoples (of course) also have a right to self-determination, and courts have recognised that for groups ranging from the Québécois to the English-speaking Cameroons. Being defined as "indigenous" is important, but not the legal be all end all.