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Clean eating. It's a term that's tagged in tweets and blogs, posted on Instagram and Facebook, and seen on television screens. Is it a diet? Is it a lifestyle? Does anyone really know what it is?

The truth is, it's a very simple concept. "In some ways clean eating is what eating was always about," said Dr.David Katz, director of the Yale University Prevention Research Center.

"Food that's clean is food that's for the most part real food and not encumbered with things that compromise health: artificial flavorings, artificial colorings, sugar substitutes," said Katz.

The clean eating rule of thumb: The shorter the ingredient list, the better. No specific food is off-limits as long as it's a real, honest-to-goodness food. In other words, this isn't a "diet" that bans bread or sacrifices sugar.

"I don't think sugar makes food unclean. Pure fruits are not unclean foods. You can add sugar to foods, and it can be clean. … It's not about banishing any particular type of ingredient," said Katz. " It needs to be a holistic concept. There's a real danger in placing it on just one ingredient."

Chef Ric Orlando, a pioneer of clean eating and the author of " We Want Clean Food," takes a more organic, local approach to clean eating. He recommends local foods because they have less impact on the environment. Clean eating for Orlando doesn't limit protein or fried foods. Natural chicken, sustainable seafood, grass-fed cow's milk are some meat options and he suggests frying with non-genetically modified oils.

Where Did It Come From?

Clean eating is mostly a new-age concept that began when we had enough cultural advancements to create excess. Books such as Michael Pollan's "The Omnivore's Dilemma" and popular documentaries like "Food, Inc." have helped bring ingredient awareness to the forefront of people's minds. Michelle Obama's healthy-eating campaign has also played a major role in this awareness.

More significantly is the need for clean eating as a necessity for health. "Heart disease, cancer, stroke and diabetes are in almost every household in the country. Those are lifestyle related conditions. We're all affected by it. The problem is so urgent at this point we can't just keep on keeping on."

Is It Any Good?

"This is a way of eating that you can eat until you're full and satisfied, and the side benefit is the weight loss.," said Ivy Larson, co-author of " Clean Cuisine." Larson's multiple sclerosis symptoms were lessened when she started eating a "clean" diet.

Although Larson and her clients have a more strict interpretation of clean eating, the core principles of the plan are the same: Eat whole foods and less packaged items.

Larson recommends starting by adding one "clean" meal a day to your diet, adding more week by week. She suggests that buying frozen vegetables or fruits is a quick way to add nutrients to your diet with less work.

A common belief is that clean eating - or healthy eating in general - is more expensive than fast-food choices. "To eat this way is actually cheaper than processed food. You just have to put in the labor," said Orlando, "We take more energy making our car nice than taking care of our bodies," he said.