There’s a few songs out there, and even a few companies with the term “Kryptonite” taken from the fictional substance from the Superman universe, by DC comics. But since it’s based on the element Krypton… perhaps it’s in the public domain?
I want to know, because I’d like to use the term commercially if possible.
ETA: You can trademark Kryptonite as long as it isn’t already trademarked in that particular area/market. For example, your bike lock company won’t be able to use the brand Kryptonite, but your shoelace company probably will.
(1) If you are planning to use the term “kryptonite” in commerce as an identifier of the source of goods or services, then you should consult a competent attorney licensed in your jurisdiction, not a message board.
(2) As Jayrot said, it seems from your description that you are talking about trademark issues, not copyright issues. Single words, names, short phrases, and titles are not protected under copyright law, but might be subject to trademark law.
(3) “Public domain” is a copyright concept. The rough equivalent of that concept in trademark law is “genericness.” You would have to ask whether “kryptonite” is a generic term for anything.
(4) A quick perusal of the Patent and Trademark Office’s Web site shows that there are several registrations for terms that include the word “kryptonite.”