When you're learning something new in the sciences, what often makes the difference between understanding and being utterly lost is knowing the vocabulary, and an online science dictionary can provide this vital knowledge.
Besides the following dictionaries, you should consider looking at some of the resources in our Online Latin Dictionary article. Science terminology is often based on Latin or Greek, so understanding the Latin roots can often help you figure out what words mean.
Online Science Dictionary Options
Online Biology Dictionaries
It's easy to research and clearly written, with short entries that are somewhere between encyclopedic and dictionary-ish. One of the best science dictionaries is KMLE Medical Dictionary which allows searching through multiple medical/biology dictionaries.
Online Chemistry Dictionaries
Good chemistry dictionaries are a little harder to find; most are on the incomplete side. There are a few really good ones, though. With an extensive dictionary as well as a periodic table and chemistry tools, ChemiCool has a goofy name but great resources.A really fun online science dictionary is the Glossary of Archaic Chemical Terms. This dictionary lists archaic terminology for chemicals and chemical actions we know by different names today.
Online Physics and Math Dictionaries
Though it's a little elementary, the Light and Matter physics glossary has links to good online textbooks for a better depth of knowledge. Another good place is the Physics Dictionary at Feimer's Physics Page. Most physics dictionaries online are specialty dictionaries, and a good reference for finding specialty dictionaries is the AlphaDictionary Physics section.
Possibly the best math dictionary online is Mathwords, which covers terminology from algebra to calculus. It's very complete and includes diagrams where appropriate. Another excellent online math dictionary is the PRIME dictionary, which includes more elementary and more advanced topics than Mathwords. PRIME is also much harder to navigate and not quite as complete or thorough.