A controller stick is a small lever that reports where it is, as a pair of numbers: how far left or right, and how far up or down. At rest it should report zero. In real life it rarely does. Springs wear, dust gets in, and even a brand-new stick can read a tiny bit off-centre.
A deadzone is a small circle around the centre where the game ignores the stick completely. Push inside the circle and nothing happens. Push past it and the game starts to respond. Its job is to stop a stick that is resting from making your camera creep or your character walk on its own. That creeping is what people call stick drift.
Most games ship with a deadzone that is a bit generous, because developers have to cope with every worn-out controller out there. A big deadzone has a cost: you have to push the stick a noticeable distance before anything moves, so tiny aim corrections feel sticky or late.
A deadzone that is too small has the opposite problem. The stick picks up its own noise, and the view drifts when you are not touching it. The goal is the smallest deadzone that still keeps a resting stick quiet.
The demo below simulates a stick. Drag inside the big circle to move it, and use the sliders to change the deadzone and to add some pretend wear. The cross-hair on the right is what the game sees.
THE STICK (drag me)
WHAT THE GAME SEES
The deadzone you just played with is the inner deadzone, around the centre. Some games also offer an outer deadzone, near the rim. It treats a push of, say, 95% as a full 100%. That helps when an old stick can no longer quite reach the edge, so you can still sprint at full speed or turn at full rate.
Notice that in the demo, once you leave the deadzone the response starts from zero again. Good games rescale the range like this so there is no sudden jump when the stick crosses the edge of the dead circle.
Look in the controller or input section of the game's settings. Names vary: deadzone, dead zone, stick threshold or stick sensitivity. Then try this:
First, put the controller down on a table and open a game or menu where you can see the view or cursor. If the view moves, your deadzone is too small or the stick is worn. Raise it a few percent at a time until everything is still.
Second, go the other way. Nudge the stick very gently while aiming at something. If you have to push before anything happens, lower the deadzone until small movements register, but stop before the resting drift returns.
Third, if the settings screen has a separate setting for the left and right stick, remember they wear differently. The right stick, used for aiming, usually deserves the smaller deadzone.
A deadzone is a small circle where the game ignores the stick, so a resting stick stays still. Smaller means more precise but more drift-prone. Larger means calmer but sluggish for tiny movements. Pick the smallest value that keeps a resting stick quiet, and if you need a very large deadzone just to stop drift, the stick itself may be worn out and worth repairing or replacing.