Lynda: Possibly "Last Common Ancestor"? Just a guess.
Marshall: I know that the WikiTree DNA Project is underway with a large effort to reorganize and rewrite all the DNA Help pages. I'm sure it's going to take some time, though.
That specific example of an mtDNA confirmation you're looking at (at least I assume that's the one) is actually incorrect. The two individuals in the example have four different common ancestors dating back as far as a 6g-grandparent, but in all four relationship instances there are males within the ancestral branches between the two mtDNA test takers, so they biologically can't share a mitochondrial DNA MRCA. I don't know of a better confirmation example to point you to at this time, however.
My own personal (unsolicited) opinion is that is mtDNA is a marvelous tool to use as evidence in a negative hypothesis (e.g., "the documented maternal line between Xg-grandmother and living female Z is incorrect") or in a population hypothesis ("the maternal line is Southeast Asian"), but as a standalone method is too weak to validate a positive statement for genealogy. Even if a paper-trail matrilineal line is correct, within the more common mtDNA haplogroups literally a million of the 7.6 billion living people will share an exact full-sequence match. The mtDNA molecule is simply too tiny to allow sufficient variation to lend itself for use as positive evidence for a family tree. And that's with a full sequence match that includes a maximum of 15,447 base pairs. The HVR1 panel tests only 546 base pairs, and HVR2 only 576. It was the first genome sequenced and the first to be used for the testing of ancient remains. But for family trees, the information it can give us is somewhat limited.