Cyclopedic Education Dictionary (netLibrary)
Greenwood Dictionary of Education (netLibrary)
Historical Dictionary of American Education (netLibrary)
American Heritage Dictionary of Idioms (netLibrary)
Learning Theories, A-Z (netLibrary)
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.