In the field of human genetics, Mitochondrial Eve refers to the matrilineal "MRCA" (most recent common ancestor). In other words, she was the woman from whom all living humans today descend, on their mother's side, and through the mothers of those mothers and so on, back until all lines converge on one person. Because all mitochondrial DNA (mtDNA) is generally passed from mother to offspring without recombination, all mitochondrial DNA (mtDNA) in every living person is directly descended from hers by definition. Mitochondrial Eve is the female counterpart of Y chromosomal Adam, the patrilineal most recent common ancestor, although they lived thousands of years apart.
Each ancestor (of people now living) in the line back to the matrilineal MRCA had female contemporaries such as sisters, female cousins, etc. but none of these female contemporaries of the "Mitochondrial Eve" has descendants living now in an unbroken female line. Mitochondrial Eve is generally estimated to have lived around 200,000 years ago, most likely in East Africa, when Homo sapiens sapiens ("anatomically modern humans") were developing as a population distinct from other human sub species. Mitochondrial Eve lived much earlier than the out of Africa migration that is thought to have occurred between 95,000 to 45,000 CE. The dating for 'Eve' was a blow to the multiregional hypothesis, and a boost to the hypothesis that modern humans originated relatively recently in Africa and spread from there, replacing more "archaic" human populations such as Neanderthals. As a result, the latter hypothesis is now the dominant one.
One of the misconceptions of mitochondrial Eve is that since all women alive today descended in a direct unbroken female line from her that she was the only woman alive at the time. Nuclear DNA studies indicate that the size of the ancient human population never dropped below tens of thousands. There may be many other women around at Eve's time with descendants alive today, but sometime in the past, those lines of descent included at least one male, who do not pass on their mother's mitochondrial DNA, thereby breaking the line of descent. By contrast, Eve's lines to each person alive today includes precisely one matrilineal line.