The definitions in our own Glossary of Grammatical and Rhetorical Terms (the most extensive language glossary online) direct you to expanded discussions elsewhere on About.com Grammar & Composition.
For most of us, learning grammatical and rhetorical terms is not an end in itself. However, if we wish to understand, discuss, and apply the principles that underlie our language, it certainly helps to have a common vocabulary.
These online glossaries provide good, clear definitions of the most common grammatical terms.
Though designed for speakers of English as a second language, this comprehensive glossary is just as useful for native speakers. Hosted by UsingEnglish.com, the site offers a "fully cross-referenced English glossary of linguistic and grammatical terms. Each grammar definition contains an explanation and cross-references to other relevant grammar terms."
Developed over the past decade by Robbin L. Simmons, the glossary at Grammar Bytes is not the largest on our list but it does contain some of the clearest explanations and most engaging examples.
Maintained by A. G. Rigg of the University of Toronto, this "old-fashioned" site aims to "provide an introductory framework for an understanding of modern English."
No bells and whistles here, but it's all that most of us will ever need: accurate definitions and clear examples.
