Dictionary of Petroleum and Oilfield Terminology: Glossary of terms. The list is searchable and also contains links to further information.
Earthquake Image Glossary: Heavily illustrated glossary of common terms in seismology.
Geological and Gem Terminology: Jillian Banfield's glossary of geological and gem-related vocabulary.
Geology Dictionary - Geology.com: Definitions of terms, written at a level appropriate for students in introductory courses.
GeologyLink Glossary: Indexed glossary of geology terms.
Glacier Glossary: An A-to-Z glossary of glacier terms written by the professor and students of an NEIU glaciology class.
Glossary of Selected Geologic Terms: Glossary of selected geological terms from the University of Nebraska-Lincoln.
Schlumberger Oilfield Glossary: Defines many terms related to oil and gas exploration and production. Includes illustrations.
USGS Photo Glossary of Volcanic Terms: Illustrates and describes commonly used volcano-related terms.
Volcanic and Geologic Terms Glossary: Common terms defined.