Specialized Scientific Dictionaries and Glossaries:

Abbreviations and Acronyms (USGS)
Agency for Toxic Substances and Disease Registry (ATSDR)
Agriculture: A Glossary of Terms, Programs, and Laws (National Council for Science and the Environment)
Aquatic, Wetland, and Invasive Plant Glossary (University of Florida)
Atmospheric Chemistry Glossary (Sam Houston State University)
Biodiversity Glossary (World Resources Institute)
Biomes Vocabulary (NASA)
Bird Dictionary (Bird On!)
Bird Glossary with photo links (USGS Patuxent Wildlife Research Center)
Botanical Glossary (with a focus on Orchids)
Botanical Terms - Searchable glossary containing 4400 terms relating to botany, gardening, horticulture and landscape architecture.  Regularly updated. (Garden Web)
Classification of Living Things Glossary (Palomar College)
Clean Air Glossary from Foundation for Clean Air Progress
Composting Terms (Cornell Center for the Environment - Cornell University)
Convention on Biological Diversity
Earth Observatory Glossary (NASA)
Earth Science Acronyms, Glossaries and Gazetteers  (Global Change Master Directory)
Eco Terms Dictionary from Eco-Web
Ecological Terms (Gov't of Canada)
Endangered Species Glossary (U.S. Fish and Wildlife Service)
Environmental Defense Fund K-6 Glossary (Cornell Waste Mgmt Institute)
(Master of Engineering student project, Southampton University)
Environmental Glossary (Nat'l Safety Council)
Environmental Protection Agency Glossary
Environmental Protection Agency Terms of Environment
"Forest Puzzles" Glossary
Forestry Terms - suggest clicking on alphabetical letter your word begins with, and looking down the list for your word and related words.  Then go back and type your word in the Search function for expanded topics on related subjects.
Genetic Terms - Talking Glossary of Genetic Terms - to help people without scientific backgrounds understand the terms and concepts used in genetic research.  click on the term of interest to open a page with a wealth of information, including the term's pronunciation, audio information, images and additional links to related terms. Students, teachers and parents will find the glossary an easy-to-use, always available learning source on genetics.
Geologic Terms (Illustrated.  Iowa State University)  Give it time to load.
Geology Glossary and Dictionary
Global Climate Research Information Office (scroll about 3/4 way down page to glossary)
Global Warming (Climate Change) Glossary (U.S. EPA)
Glossaries (University of California Museum of Paleontology, Berkeley)
Phylogenetics, Geology, Biochemistry, Cell Biology, Ecology, Life History, Zoology, Botany and Paleogeography
Glossopedia - digital library of the cultural and biological diversity on our planet.
Guide to Environmental Issues: Glossary of Terms and Acronyms (EPA)
Herbarium - Definition of a Herbarium (Harvard University)
Illustrated Dictionaries from Zoom School 
Imagine the Universe (NASA)
Insect Terms
Insectclopedia - links to glossaries and dictionaries pertaining to entomology (branch of zoology dealing with the scientific study of insects), life sciences, toxicology, organisms, chemicals, agricultural, plant pathology,  epidemiology, nematology, weed science, and terms related to various mini-beasts.  (Click on Identification/Definitions)
Life Science Dictionary - online resource containing over 8300 terms dealing with biochemistry, biotechnology, botany, cell biology and genetics. (University of Texas)
Math Glossary - Illustrated glossary with animations and diagrams to enhance student comprehension of important math terms, grades 1 through 8 (Harcourt Multimedia)
Microbiology Dictionary (bacteria, protozoa, viruses, algae, and fungi)
National Library of Medicine Search MedLinePlus or Medical Dictionary
National Wildlife Federation - Cool Tour of the Environment Glossary
Native Plants Glossary (California Native Plant Society)
Nuclear Science Terms (Lawrence Berkeley Nat'l Laboratory)
Ocean Terms Definitions (Committee for the National Institute of the Environment)
One Look - Search 993 dictionaries at once
Orchid Species Glossary
Ozone Depletion Glossary - with links  (EPA)
Pacific Institute's "Global Change" Glossary
Plants, Animals and the Environment (PAE) Glossary derived from leading WCB/McGraw-Hill textbooks in zoology, botany, environmental science and marine biology.
Prairie, Grasslands and Savannahs Glossary (Bell Museum of Natural History)
Quaternary Acronyms and Abbreviations  (geological period end of Tertiary to present time)
Reverse Dictionary - Describe a concept and get back a list of related words/phrases (One Look)
Answers questions, solve crossword puzzle clues, find words without knowing all the letters.
RhymeZone -  Search for rhymes, synonyms, definitions, quotations, Shakespeare, Mother Goose, Quizzes, Famous Documents.
Smog Glossary - (EPA/Ofc of Transportation and Air Quality - Colorado School of Mines) 
Soil Science Glossary - From the Soil Science Society of America
Sprawl/Nature in Fragments Glossary - Center for Biodiversity and Conservation
Also contains links to online resources related to sprawl, smart growth and sustainable development (near bottom of pg)
Stanford Encyclopedia of Philosophy - (enter via U.S.A, or mirror sites: Australia, The Netherlands, or UK)
Studying Earth's Environment from Space  Acronym list and glossary (NASA)
Taxonomy: What's in a Name? (Florida Center for Library Information)
Terminology Reference System - The Terminology Reference System (TRS) has been created as a single resource of environmental terminology for the Environmental Protection Agency by compiling collections of terms from EPA and other sources.
Time dating Glossary of Terms (Palomar College)
Tree Terms (Glossary) - (Prepared by Dr. Craig R. McKinley, North Carolina State University)
United Nations Environment Programme Glossary  
Units of Measurement - with conversion calculator (University of Exeter, U.K.) 
U.S. Geological Survey Glossary (USGS)
Volcanic and Geologic Terms
Water Environment Terms Water Glossary - Hydrologic definitions (USGS)
Water Environment Terms (Water Environment Federation)
Water Glossary - Environment Canada (click on alphabetical characters or scroll down)
Watershed Glossary - Purdue University
Water Words - California EPA.  Click on alphabetical characters or scroll down page.
Water Words Dictionary - Nevada Div of Water, Dept of Conservation and Natural Resources
Click on alphabetical characters.
Weather Labs Glossary - to help you better understand weather forecasts and reports.  Click on Weather glossary in left frame under Learning.
(Weather defined in hundreds of descriptive terms:  general, emotional, feels-like, marine, precipitation, sky, temperature, wind, and severe)
World Wide Words
Interesting investigation of international English from a British viewpoint. Trace the origin of phrases and weird words.