That's a good question and an important question, I believe.
I can say firmly that there is, to my knowledge, no UN body that has adopted a definition for “indigenous peoples”.
The only definition I am aware of is a 1972 working definition by Martinez Cobo, who did an important five-volume study that was given to the UN in 1978, I believe. The working definition back in 1972 was never adopted by the working group on indigenous populations, or by any UN body.
In fact, I can also add that the two international covenants that I spoke about, signed by Canada in 1976, the Covenant on Civil and Political Rights and the International Covenant on Economic, Social and Cultural Rights, both in their articles 1, talk about the right of all peoples to self-determination.
In international law, the term “peoples” was never defined; there is no definition for “peoples” either.
I think it's important to be aware of that, because once you decide to define something, then you end up excluding others. I think that's the main reason behind the UN not adopting a definition for indigenous peoples.