On July 17, 1947, The National Association of Real Estate Boards applied to the United States Patent Office to register REALTOR as a trademark. The date claimed for the first usage was March 31, 1916. It first came into common parlance with an utterance by a witness at a subcommittee hearing in 1919. Three years later, Sinclair Lewis used it in his novel, "Babbitt". The word appeared in dictionaries in 1917.
Today, most dictionaries define the word as having to do with the National Association of REALTORS®. However, this doesn't stop language from flowing around the boundaries of law. Most people make use of the word "realtor" with "real estate agent" interchangeably. In speech, it may be ignored, but since the NAR does not want individuals who are not certified by the Board to be mixed up with their own members, they are less forgiving of the improper usage of "realtor" in print, even by REALTORS® themselves.
First of all, if you're not a REALTOR® and you pass yourself off as one, with the usage of REALTOR or REALTORS in your site or your advertising, NAR is gonna get medieval on yo' assets. It usually starts with a cease-and-desist letter but gets worse from there. They aren't out to get you; they view you as a possible future REALTOR®! However, they do have the power, as the holders of the trademark, to successfully initiate legal action against those who use the word outside of their rules, even as a domain name. So, play nice.
Domain names are tricky, even for a REALTOR® in good standing. For instance, you are perfectly free to register HepzibahSnigglesworthRealtor.com, if Hepzibah Snigglesworth is really your name, but don't waste your money on ImaRealtorwhocanmakeallyourdreamscometrue.com. Also, if you try to register CrappoRealtor.com and you are not a resident of Crappo, MD, the NAR will not look upon you with a good eye. If you are a resident of Fairdealing, MO, though, you may be able to snag FairDealingRealtor.com. You can register TheBestEverRealtor.com if The Best Ever is the name of your company.
Using variations on REALTOR® is also frowned upon. Creative misspellings, terms that indicate that you are anything but a REALTOR®, even reproduction for novelty items like T-shirts can constitute infringement. You will have issues if you want to name your Great American Realtor Novel, "Realtors of Crappo". It's always good to contact the NAR when in doubt and get their advice.
One place where you might see some variations on NAR's protection of its trademarks is the news. Since journalists have their own standards to adhere to, it's possible that REALTOR® will be used without the identifying trademark symbol. Newspapers do this because trademarks break up the flow of articles and make them hard to read. However, NAR is on the lookout for articles that use realtor interchangeably with "real estate agent" or as a label for anything other than what NAR has designated it for.
When in doubt, use a synonym, euphemism, simile, metaphor or contact the NAR. The NAR isn't out to get you, but it will defend its right to set standards for its trademarks. The misuse of term REALTOR® is something that the NAR takes seriously, but with a little research, it's easy to find out what is acceptable use.