A Millésimé or Vintage Champagne is a Champagne which is composed only of a single year of harvest unlike other bottles of Champagne which are a blend of several years.
A Champagne is vintage only when it is a beautiful year or a very atypical year.
A Vintage Champagne is aged for a minimum of 36 months in the cellar compared to a minimum of 15 months for a non-vintage Champagne.