A thumbstick is a small joystick or control stick that you can use with your thumb to move or control something, often found on video game controllers. Full definition
Sure, there's a little leeway and you can move the view very slightly with the right thumbstick on the DS4 controller but sometimes that's not enough. (fingerguns.net)
It's your traditional shooter with left thumbstick for movement, right thumb for shooting, jumping, and changing weapons. (droidgamers.com)
Left click and drag or use the right thumbstick on the controller to rotate them. (gamewatcher.com)