To put it simply, a nutritionist has no concrete definition, while a dietitian has credentials to go with the term. Any person working in a health food store or otherwise can call themselves a nutritionist.

A Registered Dietitian (RD) is a credential just like a Registered Nurse (RN) or Medical Doctor (MD). To become a Registered Dietitian you must:

1. Earn a Bachelor's Degree in dietetics, a 4 year degree from an accredited college or university
2. Complete an internship with at least 900 hours
3. Take and pass the RD exam
4. Complete 50 Continuing Education Credits every 5 years to maintain license.

To earn a Bachelor's Degree, Registered Dietitians study food and nutrition sciences, foodservice systems management, business, economics, computer science, culinary arts, sociology, communications, biochemistry, physiology, microbiology, anatomy and chemistry.

A Registered Dietitian is knowledgeable in the science of nutrition. They learn how to interpret research studies and apply that knowledge to counseling individuals on how to improve their lifestyle and health. He or she is able to look at your medical history, current symptoms, medications, supplements, exercise routine, weight, and eating habits and give advice that is safe and effective for you to reach your goals.

Taken from Healthline website, written by Tara Gidus:

http://www.healthline.com/blogs/diet_nutrition/2008/05/nutritionist-vs-dietitian.html