The seventeenth letter of the modern English alphabet and the sixteenth of the ancient Roman one, in the latter an adoption of the (koppa) of some of the early Greek alphabets, in turn derived from the Phoenician letter used to represent voiced uvular.
Q was the pseudonym of the English writer and critic Arthur Quiller-Couch (1863–1944), which he originally adopted as a student at Oxford when writing parodies of English poets for the Oxford Magazine.
Q also denotes the hypothetical source of the passages shared by the gospels of Matthew and Luke, but not found in Mark; Q here probably comes from German Quelle ‘source’.
In the James Bond films, Q is the name of the elderly technician responsible for the development of Bond's customized cars and other gadgets.
See also mind one's P's and Q's.