Millennial meaning : A demographic term for a person from the generation born from around the early 1980s to the mid 1990s or early 2000s; individuals who reached adulthood early in the 3rd millennium, C.E. Referring to the 1,000th anniversary of an event or happening. Occurring every thousand years. Occurring at the end or beginning of a millennium. Of or relating to people born in the last two decades of the 20th century. Referring to the millennium, the period of one thousand years during which Christ will reign on earth.