Geology

Geological and Gem Terminology
Jillian Banfield's glossary of geological and gem-related vocabulary.
http://seismo.berkeley.edu/~jill/glossary.html

Geology Dictionary - Geology.com
Definitions of terms, written at a level appropriate for students in introductory courses.
http://geology.com/geology-dictionary.shtml

Geologylink - Glossary
Glossary covering all matters geological. Suitable for students, university instructors, and professional geologists.
http://college.cengage.com/geology/resources/geologylink/glossary.html

Glacier Glossary
An A-to-Z glossary of glacier terms written by the professor and students of an NEIU glaciology class.
http://ebeltz.net/glacier/glacglos.html

Glossary of Volcanic and Geologic Terms
Part of Vulcano World, a higher education, k-12, and public outreach project of the Oregon Space Grant Consortium administered by the Department of Geosciences at Oregon State University.
http://volcano.oregonstate.edu/education/glossary.html

Schlumberger Oilfield Glossary
Defines many terms related to oil and gas exploration and production. Includes illustrations.
http://www.glossary.oilfield.slb.com/

USGS Photo Glossary of volcanic terms
Short, but well illustrated glossary of terms related to volcanoes.
http://volcanoes.usgs.gov/images/pglossary/index.php

Visual Earthquake Glossary
Provides terms and their definitions, each with an image. Edited by the USGS (U.S. Geological Survey) Earthquake Hazards Program.
http://earthquake.usgs.gov/learning/glossary.php