The most common definition of GIS is geographic information systems. It is comprised of a full software and hardware system that can capture geographic data and information via cameras, store it via a database, manipulate it via software, analyze it via statistical and visualization tools, and manage and present spatial or geographic data. There are other well-known defination for GIS, such as geographic information science, although that has fallen out of general use as it refers to the academic discipline that studies geographic information systems. It is not commonly used for the definition of GIS, because it is a large domain within the much broader academic discipline of geoinformatics.
This essentially boils down to the ability to describe any information system that can combine, keep for future usage, manage, examine, distribute, and manifest geographic information...