Wiktionary:Word of the day/2023/October 20

1. Thousand-year-old; also (by extension, loosely) thousands of years old.
2. Occurring every thousand years.
3. Occurring at, or relating to, the beginning or end of a millennium.
4. Referring to the thousandth anniversary of an event or happening.
5. (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, and who reached adulthood early in the third millennium C.E.
6. (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”)

millennial n

1. (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.
2. (obsolete) A thousandth anniversary; also, a celebration of such an anniversary.

Today is the eve of the birth anniversary of American author and consultant Neil Howe, who was born on October 21, 1951. Howe, in collaboration with William Strauss, developed the Strauss–Howe generational theory, and coined the sense of the word referring to the generation of people born from around the early 1980s to the mid-1990s.