According to Thomas

New (perfect condition)

Pages are clean and are not marked by notes, highlighting or fold.

Like new (excellent condition)

Pages are clean and are not marked by notes, highlighting or folds.

Very good (good condition)

Pages are intact and may have minimal notes and/or highlighting or folds.

Good (clean condition)

All pages and the cover is intact. The spine may show signs of wear. Pages include notes and/or highlighting.

Acceptable (readable condition)

All pages and the cover is intact. Pages include considerable notes in pen or highlighter, but the text is not obscured.