USGS:
In the strictest sense, a GIS is a computer system capable of assembling, storing, manipulating, and displaying geographically referenced information , i.e. data identified according to their locations. Practitioners also regard the total GIS as including operating personnel and the data that go into the system.
ESRI:
GIS is a combination of
The user manipulates the hardware, which engages the software, to work on the data. Together, they handle
GIS is not "canned maps." It is, instead, the ability to construct maps "on the fly," showing what you want (and have data for) in the way you define. The software draws the integration of data: geographic coordinates (or "where things are") and sets of attributes (or "what things are like"), processed according to rules set by the user. This requires high computer power, since you can draw an infinitely variable set of coordinates at infinitely variable scales.
Volusia County, FL Geographic Information Services:
A computer system capable of holding and using data describing places on the earth's surface.
Many computer programs, such as spreadsheets, statistics packages or drafting packages can handle simple geographic or spatial data, but this does not necessarily make them a GIS. A true GIS links spatial data with geographic information about a particular feature on the map. For example, the centerline that represents a road on a map doesn't tell you much about the road except its location. To find out the road's width or pavement type, you must query the database. Using the information stored in the database, you could create a display symbolizing the roads according to the type of information that needs to be shown.
In short, a GIS doesn't hold maps or pictures - it holds a database. The database concept is central to a GIS and is the main difference between a GIS and drafting or computer mapping systems, which can only produce a good graphic output. All contemporary geographic information systems incorporate a database management system.
A GIS gives you the ability to associate information with a feature on a map and to create new relationships that can determine the suitability of various sites for development, evaluate environmental impact, identify the best location for a new facility, and so on.
A revised definition of: Geographic Information Systems, Nick Chrisman
Definition of a GIS, SAGE Introductory Guidebook, by Robert M. Itami and Robert J. Raulings
GIS: Tool or Science?, Dawn J. Wright, Michael F. Goodchild, and James D. Proctor; the Annals of the Association of American Geographers, 87(2): 346-362, 1997.
Demystifying the Persistent Ambiguity of GIS as "Tool" Versus "Science"