Flexitarian is a term recently coined to describe those who eat a mostly vegetarian diet but occasionally eat meat. Many people who call themselves flexitarian or semi-vegetarian have given up red meat for health reasons while others, for environmental reasons, only eat free-range or organic animals and animal products.
Vegetarians do not eat meat. A flexitarian or a semi-vegetarian is not a vegetarian. Many vegetarians strongly resent the use of the term flexitarian.
So, what is a flexitarian diet? Flexitarian is used to describe a diet or a person who eats a mostly vegetarian diet, occasionally including meat. There is no standard agreement or definition of what this means; whether flexitarians eat meat once a day, once a week or just occasionally is up to the individual person.
The word flexitarian has been around for a while but hit the mainstream with the publication of the book, The Flexitarian Diet in 2008. CNN, MSNBC, and Newsweek have all covered the flexitarian trend. But in true 21st-century fashion, the real signifier that flexitarians are here to stay is that they have their own Facebook group.
Pescatarian vs. Flexitarian
Flexitarianism differs from pescetarianism, which is a diet that includes the animal flesh of fish only, along with vegetarian foods. So, while a pescatarian will only eat fish, but as much fish as they'd like, a flexitarian may eat any type of meat, but only on an occasional basis.
This means that a pescatarian who occasionally eats fish may be a flexitarian, but a flexitarian is not necessarily a pescatarian, as they may be eating chicken, pork, beef and so on.
Opponents say there's no such thing as mostly vegetarian, just as there's no such thing as being mostly pregnant. By simple definition of terms, you cannot be a meat-eating vegetarian; just like you cannot create a 4-sided triangle, no matter how hard you might try.
All of the arguments in favor of adopting a flexitarian diet (health, environment, reduction of resource consumption) could be viewed as arguments in favor of adopting a fully vegetarian diet.
Flexitarianism has been popping up more and more in the media, so no longer can vegetarians just wish the word would go away. In other words, as a flexitarian, one is still taking a hugely positive step forward for health, the environment, and the animals and that's a great thing.