Maths dictionary by Jenny Eather

Math Dictionary for Kids is an excellent helper for kids all over the world.

this interactive dictionary allows users to search for terms, mark favorites for studying further, take notes on definitions and see related terms. It includes illustrated, concise explanations of the most common terms used in general math classes, categorized by alphabetical scroll list. 
The maths charts include definitions and examples covering Numbers, Operations on Numbers, Fractions, Decimals, Percent and Percentages, Ratios and Rates, Beginning Algebra, Data and Statistics, Probability, Geometry, Measurement, Time and Money.

I highly recommend it!