Maths Dictionary (http://www.amathsdictionaryforkids.com)
Maths dictionary for kids is an excellent online resource for helping children understand certain maths terms. I first used this resource as software in my school, but later on found out that it was free online. It has over 630 different mathematical explanations. There are interactive diagrams/pictures to help with the explanations.
There is a quick reference page and you can also download maths posters for your home or classroom.