Answers:
Generation Y is a term designating a cohort of people born immediately after "Generation X" and is only one of several terms used to describe roughly the same group of people. There is however no consensus as to the exact range of birth years that constitutes "Generation Y", nor whether this term is specific to North America, the Anglophone world, or people worldwide. The only consensus, by way of its relation to the term "Generation X," is that those born in Generation Y must follow Generation X. Many published academic, demographic, business, and governmental sources have used their own specific parameters for who constitutes Generation Y, but there is by no means one accepted definition. The use of the term is also controversial and synonyms are often used in discourse or in published works.
As the term "Generation X" was coined primarily to describe the post Baby Boomer generation in the United States and Canada, some people use "Generation Y" only to refer to Americans, Canadians, and other Anglophone people who were born after Generation X. Others have suggested that such regional restrictions of use are unnecessary in the ever globalizing world.
As generations are defined not by formal process, but rather by demographers, the press and media, popular culture, market researchers, and by members of the generation themselves, there is no precise consensus as to which birth years constitute Generation Y. For instance, while the periodical American Demographics typically uses 1976 to demarcate the start of Generation Y, the demographers Howe and Strauss have consistently used "the High School class of 2000", or those born in 1982 as their demarcation. While many possible years are used as the endpoint of Generation Y, the term is almost never applied to current infants, who are part of a possibly as yet unnamed generation. Due to the flexible nature of such demographic terms, two people of the same birth year can identify as either Generation X, Y, or something that follows Y, such as the New Silent Generation and neither is wrong.
Because the term Generation Y suggests "following Generation X", and because the term Generation X was originally coined as a pejorative term, use of the term Generation Y is controversial. Numerous terms (see below) have been coined as alternatives to Generation Y, or to describe subjects of the cohort. "Millennials" is a very commonly used alternative by the popular press in the United States.
If the years 1978-2000 are used, as is common in market research, then the size of Generation Y in the United States is approximately 76 million.