It's a recently coined term to describe someone who chooses to limit the amount of meat and fish they eat, so that plant-based food makes up around 70 - 80% of their diet.
It's a very healthy way to eat , of course, and if the people who eat such a diet want a label to describe themselves, fine by me.
It becomes a nonsense when it's used as an adjective before the words vegetarian or vegan. Like the word flexitarian, these two (also fairly recently coined) words have specific meanings, invented by the person or people who coined them.
Language evolves, and 'flexitarian' will find its way into a dictionary soon. It could even be a useful term; as a vegan I could presumably go to a party thrown by a flexitarian knowing at least 70% of the food available would be vegetarian.
It's a term that's been brought into disrepute on this forum though, by the actions of a group of four or five people (or anyway, 4 or 5 usernames) who claim to be flexitarian and post here to insult and abuse veg*ns, sometimes singling out one particular user for a bullying campaign
Whether or not they are actually flexitarians I don't know, but they are bullies, liars and foul-mouthed idiots.
Recently two of them, who both use the word 'vegan' in their user names in order to confuse (FlexiVegan and FlexitarianVegan) have said , on a forum they KNOW is read mainly by young and impressionable people, that 'true' vegans will allow themselves to die in the event of serious illness or accident, rather than accept medication or procedures that have been tested on animals. Could they sink much lower?
Any hostility you find to flexitarians on this board is because of them. There are some intelligent, articulate and courteous meat-eaters who use this board too.