The Collaborative International Dictionary of English v.0.48:

Alderman \Al"der*man\ ([add]l"d[~e]r*man), n.; pl. Aldermen.
   [AS. aldormon, ealdorman; ealdor an elder + man. See Elder,
   n.]
   1. A senior or superior; a person of rank or dignity. [Obs.]
      [1913 Webster]

   Note: The title was applied, among the Anglo-Saxons, to
         princes, dukes, earls, senators, and presiding
         magistrates; also to archbishops and bishops, implying
         superior wisdom or authority. Thus Ethelstan, duke of
         the East-Anglians, was called Alderman of all England;
         and there were aldermen of cities, counties, and
         castles, who had jurisdiction within their respective
         districts.
         [1913 Webster]

   3. One of a board or body of municipal officers next in order
      to the mayor and having a legislative function. They may,
      in some cases, individually exercise some magisterial and
      administrative functions.
      [1913 Webster]

WordNet (r) 3.0 (2006):

alderman
    n 1: a member of a municipal legislative body (as a city
         council); "aldermen usually represent city wards"

Moby Thesaurus II by Grady Ward, 1.0:

54 Moby Thesaurus words for "alderman":
   MP, Member of Congress, Member of Parliament, archon, assemblyman,
   bailie, burghermaster, burgomaster, cabinet member,
   cabinet minister, chancellor, chosen freeholder, city councilman,
   city father, city manager, commissar, commissioner, congressman,
   congresswoman, councillor, councilman, councilwoman,
   county commissioner, county supervisor, elder, floor leader,
   headman, induna, lawgiver, lawmaker, legislator, lord mayor,
   magistrate, maire, majority leader, mayor, minister,
   minister of state, minority leader, party whip, portreeve, reeve,
   representative, secretary, secretary of state, selectman, senator,
   solon, state senator, supervisor, syndic, undersecretary, warden,
   whip



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

ALDERMAN. An officer, generally appointed or elected in towns corporate, or
cities, possessing various powers in different places.
     2. The aldermen of the cities of Pennsylvania, possess all the powers
and jurisdictions civil and criminal of justices of the peace. They are
besides, in conjunction with the respective mayors or recorders, judges of
the mayor's courts.
     3. Among the Saxons there was an officer called the ealderman.
ealdorman, or aldernwn, which appellation signified literally elderman. Like
the Roman senator, he was so called, not on account of his age, but because
of his wisdom and dignity, non propter oetatem sed propter sapientism et
dignitatem. He presided with the bishop at the scyregemote, and was, ex
officio, a member of the witenagemote. At one time he was a military
officer, but afterwards his office was purely judicial.
     4. There were several kinds of aldermen, as king's aldermen, aldermen
of all England, aldermen of the county, aldermen of the hundred, &c., to
denote difference of rank and jurisdiction.



The Devil's Dictionary (1881-1906):

ALDERMAN, n.  An ingenious criminal who covers his secret thieving
with a pretence of open marauding.