A polymath is someone who has a very extensive knowledge of a wide range of topics. Unlike a generalist, who knows a little bit about a lot of things, a polymath knows a great deal about a number of things. You may also hear a polymath described as a Renaissance Man (or Woman), or homo universalis, a term which was used at the time of the Renaissance. Some notable polymaths from history include: Galileo, Copernicus, Nasir al-Din al-Tusi, Su Song, Leonardo Da Vinci, Thomas Jefferson, Imhotep, and Hildegard.
The term "polymath" comes from the Greek roots poly-, for "many," and manthanein, "to learn." A polymath, in other words, is someone who has learned much. People have been using this term since the 1600s, when the Renaissance in Europe triggered a redevelopment in interest in classical learning, and society began to prize people with a number of skills and a wide knowledge base.
Polymaths are not just knowledgeable about a wide range of topics; they also have an assortment of skills. For example, a polymath might be skilled on several musical instruments, and he or she might also be a talented visual artist. Polymaths often have athletic skills as well, in sports like fencing, horseback riding, and so forth. History, the law, the sciences, literature, and a wide variety of topics may all be in the purview of a polymath, and he or she may perform independent research or experiments to learn more about the fields in which he or she is interested.
In Classical Greece, Rome, and China, the polymath was highly valued. Members of the aristocracy worked very hard to have an extensive knowledge of a range of topics, retaining specialized tutors to educate them and engaging in dialog and information exchanges with other members of educated society. The tradition of well-rounded scholarship has persisted in China to this day; in Europe, it experienced a brief period of suppression during the Dark Ages, until the Renaissance, when learning came into vogue again.
Many polymaths start out very young, and some people argue that the ability to become a polymath is a born, rather than an acquired, trait. In order to become a polymath, someone must have the drive to acquire information, and the skills to do it quickly and efficiently. Some individuals lack the intellectual potential or the time to turn into true polymaths, and they may be forced to settle for reputations as generalists; both polymaths and generalists can be useful people to have around, as they are a lot like walking encyclopedias.