They (n. third person plural) - An unidentified, unqualified, unquantified group of people who are often invoked during conversation as an authoritative body to support an idea regardless of how bizzar the idea might be. The word is most often found in the phrase "they say . . ." then proceeded by a statement with an unknown truth value associated with it and an unknown authoritative weight as no one know who 'they' really is. 'They' can range anywhere from professors, institutes, universities, celebrities, bums on the street, a TV show, a work of fiction, a magazine, a friend and everything in between and beyond what was just mentioned. 'They' might even refer to no person or no collective body of people, but in fact is a figment of the imagination where 'they' really translates into the phrase, "in my opinion". 'They' is often an opinion and can be nothing more than an opinion becuase the individual flinging about the authoritative 'they' has not done the research enough into the subject at hand to be informed enough to make a real statement supported by real facts. So, instead, the 'they-sayer', wanting to sound authoritative, well read, and smart will appeal to the mysterious 'they'.
Similar phrases used in a similar manner: "Some say", "Some people say", "It is said", "they've said", and "NPR said".