vitamin (molecular entity) organonitrogen compound organooxygen compound B vitamin Any member of the group of eight water-soluble vitamins originally thought to be a single compound (vitamin B) that play important roles in cell metabolism. The group comprises of vitamin B<small>₁</small>, B<small>₂</small>, B<small>₃</small>, B<small>₅</small>, B<small>₆</small>, B<small>₇</small>, B<small>₉</small>, and B<small>₁₂</small> (Around 20 other compounds were once thought to be B vitamins but are no longer classified as such). Any member of the group of eight water-soluble vitamins originally thought to be a single compound (vitamin B) that play important roles in cell metabolism. The group comprises of vitamin B1, B2, B3, B5, B6, B7, B9, and B12 (Around 20 other compounds were once thought to be B vitamins but are no longer classified as such).