ArtLex  - dictionary of art terminology, especially for art students elementary through undergraduate.

Dictionary of Philosophy of Mind  - dictionary for those terms commonly used, defined, contested, and misunderstood in the philosophy of mind. Submissions welcome.

Art Movements and Periods - brief descriptions of major art movements.

Grove Dictionary of Art Online - subscription service including the complete, fully searchable text with images.

Textile Dictionary - definitions of cotton, silk, specialty, and plant fibers.

Words of Art: An On-line Glossary of Theory and Criticism for the Visual Arts