CQC (Close Quarters Combat) is a system of combined combat techniques which allow rapid alternation between armed and hand-to-hand combat while engaging enemy personnel.
In the typical CQC stance, the user holds their gun (a one handed weapon such as a handgun) with their dominant hand while supporting it with the other hand, which holds their CQC knife. By using this stance, the gun remains steady for accurate engagement of opponents outside the range of unarmed combat, while allowing a quick transition to the knife for close-work.
In the Metal Gear Solid universe it's a style of close combat techniques developed together by The Boss and Big Boss of Metal Gear Solid 3: Snake Eater.
Use this tag for when a character in the picture is wielding a gun and a knife.