Online Math Dictionaries

All three dictionaries have different visual representations. These can be used simultaneously to compare definitions, or the learners can pick the one which they visually find more appealing. The last two links house printable lists and flashcards. 
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A Maths Dictionary for Kids
This is an animated, interactive, online math dictionary by Jenny Eather, which explains over 600 common mathematical terms in simple language. The students read the definition, see the animated examples (where needed), apply their knowledge by solving some question in for certain terms. An excellent tool for students to use at home, and teachers to use at school with white boards. Parents can share this resource at all times while helping their child acquire new math concepts.  
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Coolmath's Online Dictionary
This online dictionary is designed as a bingo grid template. The definitions include still graphics, and hyperlinks to the words that the learner might not know in order to understand the definition. This online dictionary is useful when following a math lesson on Cool Math.com or playing games on Cool Math - Games site. 
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Illustrated Math Dictionary
As the name suggests, this dictionary by Math is Fun.com shows the word definitions, as well as corresponding illustrations and animations (a few), which are helpful for visual learners. When you click on the "more" section, you will find relevant examples that explain the term. This online dictionary houses 721 definitions.  

Printable word lists & flashcards

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Math Open Reference 
This project is a work in progress, and is a great reference resource for geometry. Although it is focused on secondary education, but it can be used from grade 5 upwards.
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Granite School District Math Vocabulary
Here you will find printable lists of math vocabulary based on the common core curriculum. The primary lists consist of color illustrations along with the definitions and examples.