Q Branch (or later in the franchise Q Division) is the fictional research and development division of the British Secret Intelligence Service (SIS/MI6). The branch is dedicated to developing tools for agents to use on the field.
In the Fleming novels there are frequent references to 'Q branch', a division of the British Secret Service which provides equipment to field operatives. There is a reference to Q in Chapter 3 of Casino Royale, the first Bond book, when M says to Bond: "Go over a few days before the big game starts and get your hand in. Have a talk to Q. about rooms and trains, and any equipment you want." There is a similar reference in the following novel, Live and Let Die. However, this appears to be an abbreviation for Q Branch rather than the codename for a particular individual.