The Collaborative International Dictionary of English v.0.48:

Party \Par"ty\ (p[aum]r"t[y^]), n.; pl. Parties
   (p[aum]r"t[i^]z). [F. parti and partie, fr. F. partir to
   part, divide, L. partire, partiri. See Part, v.]
   1. A part or portion. [Obs.] "The most party of the time."
      --Chaucer.
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   2. A number of persons united in opinion or action, as
      distinguished from, or opposed to, the rest of a community
      or association; esp., one of the parts into which a people
      is divided on questions of public policy.
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            Win the noble Brutus to our party.    --Shak.
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            The peace both parties want is like to last.
                                                  --Dryden.
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   3. A part of a larger body of company; a detachment;
      especially (Mil.), a small body of troops dispatched on
      special service.
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   4. A number of persons invited to a social entertainment; a
      select company; as, a dinner party; also, the
      entertainment itself; as, to give a party.
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   5. One concerned or interested in an affair; one who takes
      part with others; a participator; as, he was a party to
      the plot; a party to the contract.
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   6. The plaintiff or the defendant in a lawsuit, whether an
      individual, a firm, or corporation; a litigant.
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            The cause of both parties shall come before the
            judges.                               --Ex. xxii. 9.
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   7. Hence, any certain person who is regarded as being opposed
      or antagonistic to another.
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            If the jury found that the party slain was of
            English race, it had been adjudged felony. --Sir J.
                                                  Davies.
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   8. Cause; side; interest.
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            Have you nothing said
            Upon this Party 'gainst the Duke of Albany? --Shak.
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   9. A person; as, he is a queer party. [Now accounted a
      vulgarism.]
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   Note: "For several generations, our ancestors largely
         employed party for person; but this use of the word,
         when it appeared to be reviving, happened to strike,
         more particularly, the fancy of the vulgar; and the
         consequence has been, that the polite have chosen to
         leave it in their undisputed possession." --Fitzed.
         Hall.
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   Party jury (Law), a jury composed of different parties, as
      one which is half natives and half foreigners.

   Party man, a partisan. --Swift.

   Party spirit, a factious and unreasonable temper, not
      uncommonly shown by party men. --Whately.

   Party verdict, a joint verdict. --Shak.

   Party wall.
      (a) (Arch.) A wall built upon the dividing line between
          two adjoining properties, usually having half its
          thickness on each property.
      (b) (Law) A wall that separates adjoining houses, as in a
          block or row.
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WordNet (r) 3.0 (2006):

party wall
    n 1: a wall erected on the line between two properties and
         shared by both owners

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

PARTY WALL. A wall erected on the line between two adjoining estates,
belonging to different persons, for the use of both estates. 2 Bouv. Inst.
n. 1615.
     2. Party walls are generally regulated by acts of the local
legislatures. The principles of these acts generally are, that the wall
shall be built equally on the lands of the adjoining owners, at their joint
expense, but when only one owner wishes to use such wall, it is built at his
expense, and when the other wishes to make use of it, he pays one half of
its value; each owner has a right to place his joists in it, and use it for
the support of his roof. When the party wall has been built, and the
adjoining owner is desirous of having a deeper foundation, he has a right to
undermine such wall, using due care and diligence to prevent any injury to
his neighbor, and having done so, he is not answerable for any consequential
damages which may ensue. 17 John. R. 92; 12 Mass. 220; 2 N. H. Rep. 534.
Vide 1 Dall. 346; 5 S. & R. 1.
     3. When such wall exists between two buildings, belonging to different
persons, and one of them takes it down with his buildings, he is required to
erect another in its place in a reasonable time, and with the least
inconvenience; the other owner must contribute to the expense, if the wall
required repairs, but such expense will be limited to the costs of the old
wall. 3 Kent, Com. 436. When the wall is taken down, it must be done with
care; but it is not the duty of the person taking it down to shore up or
prop the.,house of his neighbor, to prevent it from falling; if, however,
the work be done with negligence, by which injury accrues to the neighboring
house, an action will lie. 1 Moody & M. 362. Vide 4 C. & P. 161; 9 B. & C.
725; 12 Mass. R. 220; 4 Paige's R. 169; 1 C. & J. 20; 1 Pick. 434; 12 Mass.
220; 2 Roll., Ab. 564; 3 B. & Ad. 874; 2 Ad. & Ell. 493 Crabb on R. P. Sec.
500. In the excellent treatise of M. Lepage, entitled "Lois des Batimens,"
part 1, c. 3, s. 2, art. l, will be found a very minute examination of the
subject of party walls, with many cases well calculated to illustrate our
law. See also Poth. Contr. de Societe, prem. app. n. 207; 2 Hill.: Ab. 119;
Toull. liv. 2, t. 2, c. 3.