ESRI GIS Dictionary: http://support.esri.com/index.cfm?fa=knowledgebase.gisDictionary.gateway

Mathworld: http://mathworld.wolfram.com/

NIST Dictionary of algorithms and data structures: http://www.nist.gov/dads/

University of California GIS Abbreviations Dictionary: http://www.lib.berkeley.edu/EART/abbrev2.html#index