What is Ancestor?

Legal Definition
An ancestor or forebear is a parent or (recursively) the parent of an ancestor (i.e., a grandparent, great-grandparent, great-great-grandparent, and so forth). Ancestor is "any person from whom one is descended. In law the person from whom an estate has been inherited."

Two individuals have a genetic relationship if one is the ancestor of the other, or if they share a common ancestor from the past. In evolutionary theory, species which share an evolutionary ancestor are said to be of common descent. However, this concept of ancestry does not apply to some bacteria and other organisms capable of horizontal gene transfer.

Assuming that all of an individual's ancestors are otherwise unrelated to each other, that individual has 2n ancestors in the nth generation before him and a total of about 2g+1 ancestors in the g generations before him. In practice, however, it is clear that the vast majority of ancestors of humans (and indeed any other species) are multiply related (see pedigree collapse). Consider n = 40: the human species is more than 40 generations old, yet the number 240, approximately 1012 or one trillion, dwarfs the number of humans who have ever lived.

Some cultures confer reverence to ancestors, both living and dead; in contrast, some more youth-oriented cultural contexts display less veneration of elders. In other cultural contexts, some people seek providence from their deceased ancestors; this practice is sometimes known as ancestor worship or, more accurately, ancestor veneration.
-- Wikipedia
Legal Definition
Descents. One who has preceded another in a direct line of descent; an ascendant. In the common law, the word is understood as well of the immediate parents, as, of these that are higher; as may appear by the statute 25 Ed. III. De natis ultra mare, and so in the statute of 6 R. III. cap. 6, and by many others. But the civilians relations in the ascending line, up to the great grandfather's parents, and those above them, they term, majores, which common lawyers aptly expound antecessors or ancestors, for in the descendants of like degree they are called posteriores. Cary's Litt.45. The term ancestor is applied to natural persons. The words predecessors and successors, are used in respect to the persons composing a body corporate. See 2 Bl. Com. 209; Bac. Abr. h. t.; Ayl. Pand. 58.
-- Bouviers Law Dictionary
Legal Definition
One who has preceded another in a direct line of descent; a lineal ascendant. A former possessor; the person last seised. Termes de la Ley; 2 Bl. Comm. 201. A deceased person from whom another has inherited land. A former possessor. Bailey v. Bailey, 25 Mich. 185; McCarthy v. Marsh, 5 N. Y. 275; Springer v. Fortune, 2 Handy, (Ohio,) 52. In this sense a child may be the "ancestor" of his deceased parent, or one brother the "ancestor" of another. Lavery v. Egan, 143 Mass. 389, 9 N. E. 747; Murphy v. Henry, 35 Ind. 450. The term differs from "predecessor," in that it is applied to a natural person and his progenitors, while the latter is applied also to a corporation and those who have held offices before those who now fill them. Co.. Litt 78b.
-- Black's Law Dictionary
Legal Definition
The relative of a particular individual from whom that individual is descended directly.
Legal Definition
One from whom an inheritance is claimed. See 29 L. R. A. 542, note.
-- Ballentine's Law Dictionary