The term "fictitious business name" (or "assumed business name," "trade name," or "DBA" for "doing business as") is used when a business uses a name that's different from its legal name. For instance, if John O'Toole names his sole proprietorship Turtle's Classic Cars, the name "Turtle's Classic Cars" is a fictitious business name because it does not contain John's last name, "O'Toole."
If your business uses a fictitious business name, you'll need to register it with a government agency -- in most states, your local county clerk's office.
For more information, read Registering Your Business Name.
If you need legal advice for your business, talk to a lawyer.