Yes, you can use the phrase "real estate" in your domain name.
You should be careful to avoid the use any word or phrase in a domain name that is a registered trademarks. A good first step is to search the trademark database at the United States Patent and Trademark Office website. Consult your attorney if you have any questions.
If you're a real estate agent, one registered mark to be aware of is "Realtor". This word is a registered trademark of the National Association of Realtors (NAR). You must be an NAR member to use "Realtor" in any way. Even if you are a member, there are still rules about how you can use the term and logo. See NAR's Logo and Trademark FAQ for more information.
