It means "millenary, relative to the millennium". It is also a name for the generation born after 1980 and who reached adulthood in the next millennium.
It is the English name for Generation Y, people born in the 1980s who became adults in the new century. It is castellanized as millennial. See English/millennial .
Anglicism for millennial, milenico, who was born around the year 2000 or in the last decades of the 20th century, young who have grown up with the internet, mentality and behavior, open and free.