There are a number of Computer Science and GIS glossaries on the Web.

N.B. A problem for GIS students and professionals is that the terminology of GIS is not fixed. What is an 'arc' in one person's vocabulary may be a 'segment' in another's and an 'edge' in another's. Do not be surprised if the glossaries do not always agree!

On the other hand, looking up terms in a glossary can be an efficient way of clearing up misunderstandings.

AGI Dictionary	The revised AGI online GIS dictionary includes definitions for 980 terms compiled from a variety of sources which either relate directly to GIS or which GIS users may come across in the course of their work. The dictionary is also supplemented by 52 diagrams. (Currently unavailable due to server failure - 18.09.08)
Computing Dictionary	The 'Free Online Dictionary of Computing'.
ESRI Library - GIS Glossary	A GIS Glossary authored by G. Padmanabhan, Mark R. Leipnik, Jeawan Yoon, 1992. Downloadable as a PDF document. Provides a useful, if ESRI-oriented, set of GIS terms.
GIS Dictionary - ESRI	This dictionary includes terms from GIS operations such as analysis, data management, and geocomputation; from rapidly evolving uses of GIS for modeling, GIScience, and Web-based GIS; and from the GIS foundation fields of cartography, spatial statistics, computer science, surveying, geodesy, and remote sensing. This online GIS Dictionary also includes ESRI software-specific terminology.
GITA	Glossary of GIS terms.
PC Webopaedia	Another online computer science glossary site.
whatis.com	The 'Whatis.com' site provides a Computer Science glossary of terms. Some of the descriptions run to a page or more. You can learn a lot by searching this site.