USGS Paleontology - Glossary of Terms
USGS

Volcanic and Geologic Terms
Volcano World, University of North Dakota

Seismology Glossary
Southern California Earthquake Data Center

Glossary of Climate Terms
CalSpace

Water Science Glossary of Terms
USGS Water Science for Schools

Volcanic and Geologic Terms
Volcano World, University of North Dakota

Oilfield Glossary
Schlumberger

Internet Glossary of Soil Science Terms
Soil Science Society of America

Glossary of Meteorology
American Meteological Society

Climate and Weather Terms Glossary
Western Regional Climate Center at the Desert Research Institute, NOAA

JetStream: Glossary
National Weather Service Online School for Weather, NOAA

Glossary of Physical Oceanography and Related Disciplines
Stephen K. Baum, Texas A&M University

Glossary of Coastal Terminology
Coastal & Hydraulics Laboratory, US Army Corps of Engineers

GIS Dictionary
GIS Lounge

WordNet
"WordNet® is an online lexical reference system whose design is inspired by current psycholinguistic theories of human lexical memory. English nouns, verbs, adjectives and adverbs are organized into synonym sets, each representing one underlying lexical concept. Different relations link the synonym sets."
George A. Miller, Cognitive Science Laboratory, Princeton University

Oxford English Dictionary Online
UCSD and subscriber access only.

Merriam-Webster Online

Word Central

The American Heritage^® Dictionary of the English Language