Can I register and used a domain name that is a word trademarked by a company? An example might be “delta” (Delta Airlines or Delta Faucets) or “united” (United Airlines or various football teams).

Yes. Anyone can register a trademarked word or word combination so long as the registrant does not use the domain name to infringe on the rights of the trademark holder.

Trademark infringement is the unauthorized use of a trademark or service mark on or in connection with goods and/or services in a manner that is likely to cause confusion, deception, or mistake about the source of the goods and/or services.

More plainly, the registrant cannot use the domain, the registered word, to hold themselves out as associated with or representing the owner of the trademark or take advantage of the trademark owners’ position in the business community. So, the registrant of delta.art might feature the changing nature of the art marketplace but not beautifully designed faucets or airplanes (absent some agreement with trademark owner).

Mere registration of a trademarked name does not automatically make a case for infringement. In another plain example, it is easy to think of many uses for the words “delta” and “united,” so mere registration of those words does not give rise to suspicion of infringement. However, the words “microsoft” or “twitter” have few (if any) uses other than the trademarked purpose and their mere registration is likely to raise a case for infringement.