Legal Question in Intellectual Property in California

Re: Did I Plagiarize and Am I Liable?

Plagiarism is a form of copyright infringement, and whether it has occurred depends upon the language of the copyright act and interpretations thereof. Material that is in the public domain isn't subject to copyright, and this could include most simple, basic definitions of words. However, I expect extended definitions could be copyrighted.

As to material taken but source-cited, this also is not likely to be plagiarism, but easily could be garden-variety copyright infringement, since mere citation of source is insufficient; you also need permission for extensive quotation beyond a "fair use" limit.

In your situation, it appears that the issue is whether you breached your contract with your publisher; this may or may not turn upon whether there was copyright infringement of the plagiarism sort or of any other kind. What is controlling in your case would be whether you did what the contract called for, or whether, on the other hand, you took impermissible shortcuts and left the publisher in jeopardy of being sued for infringement itself.

No attorney could properly advise you or give you an assessment of your risk without reviewing your contract and, probably, comparing your work with the works you drew from. I would suggest that you find a copyright lawyer near you and obtain advice as to how to reply to the threats of suit. The lawyer should look at whether you are liable for breach of contract with your publisher and also whether you have possible tort liability to the third-party authors and publishers you drew from.