What is Alderman?

Legal Definition
An alderman is a member of a municipal assembly or council in many jurisdictions founded upon English law. The term may be titular, denoting a high-ranking member of a borough or county council, a council member chosen by the elected members themselves rather than by popular vote, or a council member elected by voters.

The title is derived from the Old English title of ealdorman, literally meaning "elder man", and was used by the chief nobles presiding over shires.

Similar titles exist in Germany and in Nordic countries, such as the German Ältester, the Swedish Ålderman, the Finnish Oltermanni and the Danish Olderman, which all mean "elder man" or "wise man".
-- Wikipedia
Legal Definition
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 ibe 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 bisbop 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 judical.

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.
-- Bouviers Law Dictionary
Legal Definition
A judicial or administrative magistrate. Originally the word was synonymous with "elder," but was also used to designate an earl, and even a king. In English law. An associate. to the chief civil magistrate of a corporate town or city. In American cities. The aldermen are generally a legislative body, having limited judicial powers as a body, as in matters of internal polico regulation, laying out and repairing streets, constructing sewers, and the like; though in many cities they hold separate courts, and have magisterial powers to a considerable extent Bouvier.
-- Black's Law Dictionary
Legal Definition
An associate of the chief civil magistrate of an English city or town. A member of the governing board of a city. See 4 Hill (N. Y.), 384.
-- Ballentine's Law Dictionary