Online dictionaries.The two mentioned below complement each other, and are free!Visual Mathematics Dictionary - includes definitions of terms, often with pictures, and links to related terms, which I think is a very nice feature.A Maths Dictionary For Kids 2007 - this one explains the term and often has some kind of interactive activitiy you can do right on the site, such as converting Celsius to Fahrenheit, or draw triangles, color or click things.