Pictures can help to learn and practice new words. Try these picture dictionaries to find one you like:
http://www.visualdictionaryonline.com/ (labeled pictures sorted by topic)
http://www.vidtionary.com/ (videos showing what words mean)
https://www.nasa.gov/audience/forstudents/k-4/dictionary/index.html (science-specific, more advanced, gives example sentences)
https://www.mathsisfun.com/definitions/letter-a.html (math-specific, advanced, does have some pictures)