Some math dictionaries I like:
- Math Dictionary at Coolmath.com
Aimed at K-Algebra students. Uses lots of color.
- Illustrated Mathematics Dictionary at MathIsFun.com
Very nicely done, for K to intro to Calculus. Definitions use simple language. Lots of nice interactive illustrations.
- Mathwords.com
Covers beginning Algebra through Calculus. Definitions are written at a higher reading level.
See this blog post of Concordia University - Portland for a more thorough description of each of the above.
For an in-depth treatment of any topic, see Wikipedia. They do a good job of pretty much everything, for free. If you use it as much as I do, I encourage you to consider making a donation every now and then.