adj

• (by extension, originally US, demography) Often capitalized: of or relating to, or characteristic of, people born in the last two decades of the 20th century from around the early 1980s to the mid 1990s (with 1996 births cited as the last Millennial year), and who reached adulthood early in the third millennium C.E.
  "the millennial generation"
• (Christianity) Synonym of millenarian (“pertaining to the belief in an impending period of one thousand years of peace and righteousness associated with the Second Coming of Christ and his reign on earth”).
  "the millennial judgment"

noun

• (originally US, demography) Often capitalized: a person from the generation born in the last two decades of the 20th century, from around the early 1980s to the mid-1990s and who reached adulthood early in the third millennium C.E., characterized by their familiarity with computer technology and poorer financial prospects than their parents.
  "Someday, Boomers hope, Millennials will build according to great ideals their parents can only envision, act on vital issues their parents can only ponder."
• (obsolete, rare) A thousandth anniversary; also, a celebration of such an anniversary.
  "The Magyar celebrates the millennial of Hungary, not of Austro-Hungary."