Which definition best applies to the term cultural assimilation?

the adoption of a brand new ideology by all groups within a society the adoption by a smaller group of a larger group’s beliefs, ideas, and practices
the adoption of elements of another society’s culture in order to resist colonization
the adoption of the culture of a colonized people by a colonizer

The second definition, "the adoption by a smaller group of a larger group’s beliefs, ideas, and practices", best applies to the term cultural assimilation.