The Collaborative International Dictionary of English v.0.48:

Ancestor \An"ces*tor\, n. [OE. ancestre, auncestre, also
   ancessour; the first forms fr. OF. ancestre, F. anc[^e]tre,
   fr. the L. nom. antessor one who goes before; the last form
   fr. OF. ancessor, fr. L. acc. antecessorem, fr. antecedere to
   go before; ante before + cedere to go. See Cede, and cf.
   Antecessor.]
   1. One from whom a person is descended, whether on the
      father's or mother's side, at any distance of time; a
      progenitor; a fore father.
      [1913 Webster]

   2. (Biol.) An earlier type; a progenitor; as, this fossil
      animal is regarded as the ancestor of the horse.
      [1913 Webster]

   3. (Law) One from whom an estate has descended; -- the
      correlative of heir.
      [1913 Webster]

WordNet (r) 3.0 (2006):

ancestor
    n 1: someone from whom you are descended (but usually more
         remote than a grandparent) [syn: ancestor, ascendant,
         ascendent, antecedent, root] [ant: descendant,
         descendent]

Moby Thesaurus II by Grady Ward, 1.0:

47 Moby Thesaurus words for "ancestor":
   ancestress, announcer, antecedent, ascendant, avant-garde,
   begetter, bellwether, buccinator, bushwhacker, explorer, forebear,
   forefather, foregoer, forerunner, front runner, frontiersman,
   fugleman, grandparent, groundbreaker, guide, harbinger, herald,
   innovator, lead runner, leader, messenger, parent, pathfinder,
   pioneer, point, precedent, precursor, predecessor, premise,
   primogenitor, procreator, progenitor, progenitress, progenitrix,
   prototype, scout, stormy petrel, trailblazer, trailbreaker,
   vanguard, vaunt-courier, voortrekker



Bouvier's Law Dictionary, Revised 6th Ed (1856):

ANCESTOR, 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.