Is there a term for the group of individual organisms whose matrilineally descended female ancestors all share a common ancestor? A group of individual organisms that are all descended from a common ancestor is a clade. Is there a term for a group of individuals whose matrilineally descended ancestors (mothers, maternal grandmothers, maternal grandmothers' mothers, etc.) are all descended from a common ancestor?

I'm not aware of any one-word term for this. The term "common mitochondrial ancestor" is often used to refer to a shared female ancestor of a genetically related group. If you want to specify female descendants only, perhaps you could say "females sharing a common mitochondrial ancestor". I think that's about the shortest way to say it.