Some free math dictionaries available online are iCoachMath's dictionary, A Maths Dictionary for Kids, EasyCalculation.com's dictionary and Math Is Fun's dictionary. These online resources have terms for mathematics mainly targeted to schoolchildren.
ICoachMath.com features a categorized dictionary separating terms for elementary math, middle school math and high school math with a section for important formulas. Not only does iCoachMath provide a glossary of terms, but also a concrete lesson plan for children. The resource has sample test questions and answers as well as homework help.
A Maths Dictionary for Kids is an animated, interactive tool created by Jenny Eather. This dictionary is restricted to elementary school level math but has fun and engaging methods for young kids to get interested in the subject matter.
EasyCalculation.com provides a math dictionary solely containing the essential terms. This resource has thousands of important math terms, but does not have any kind of lesson plan or organizational structure for learning said terms.
Math Is Fun also has an online dictionary available. This is a simplified dictionary with important terms for elementary and middle school students. Not only does it have the terms organized alphabetically, but the website also provides an illustrated dictionary for more visual learners.