Semi-vegetarianism is a term used to describe diets that are not vegetarian, but include less meat than typical diets. The term has no precise or widely accepted definition, but is generally defined as the avoidance of red meat and/or generally following a vegetarian diet, but eating meat occasionaly. The term is sometimes used interchangeably with "flexitarianism".
    Semi-vegetarian and flexitarian have been dubbed "problematic" and "diametrically opposed to vegetarianism" by critics.
    Flexitarians avoid, but occasionally eat, meat. In 2003, the American Dialect Society voted flexitarian  as the year's most useful word and defined it as "a vegetarian who occasionally eats meat".
    Pollotarians eat chicken and other poultry, but not mammalian meat, fish, or seafood.
    Pescetarians eat fish and seafood, but not mammalian meat or poultry.

source : http://en.wikipedia.org/wiki/Flexitarianism

cf. aussi en français : http://fr.wikipedia.org/wiki/Flexitarisme
    Le flexitarisme est la pratique d'être flexible concernant le degré avec lequel on pratique le végétarisme ou le véganisme. Un flexitarien peut manger végétarien à la maison uniquement, mais manger des plats incluant de la viande chez de la famille ou des amis.