American Heritage Dictionary of the English Language
Cyclopedic Education Dictionary (netLibrary)
Greenwood Dictionary of Education (netLibrary)
Historical Dictionary of American Education (netLibrary)
Merriam Webster Dictionary and Thesaurus
American Heritage Dictionary of Idioms (netLibrary)
Learning Theories, A-Z (netLibrary)
Merriam-Webster's Online Dictionary
OED: Oxford English Dictionary (OUP) - The largest dictionary of word descriptions and history of usage. Also available in print in Main Library Reference -- PE1625.M7 1989.