The word hello has become a ubiquitous and sometimes even innocuous part of our English-speaking world. It can say so much and so little. Hello's meaning varies with circumstance and tone. It begins more conversations than any other word and is one of few words that stand alone as a complete sentence. Despite how common and useful it is, it's relatively new.
"Millennial" in a generational sense isn't a popular word, especially with Millennials. It eventually won out as the term of use for people born between 1982 and 2004 over "Generation Y" "Generation Next" and "Baby Boomlets" likely because all of these other terms sucked just as bad.