Input lag is the time that passes between when you press a button on your controller or keyboard and when the action appears on your monitor. It's measured in milliseconds. Every device in the chain adds a tiny delay: your input device, the console or PC, the game engine, and your display.
This matters most in competitive games where split-second reactions win matches. A fighting game, shooter, or rhythm title with high input lag feels sluggish and unfair, even if the frame rate is smooth. You might press jump a fraction too late because the screen hasn't caught up with your input.
Real-world example: in a fast-paced shooter, 100ms of input lag means your aim responds a tenth of a second after you move the mouse. Your opponent with a 20ms setup will feel snappier and have a genuine advantage.
Input lag comes from several sources. Wireless controllers add delay compared to wired ones. Displays with slower response times (especially older LCD TVs) stack on extra milliseconds. Some games optimise better than others. Even the USB polling rate of your mouse affects how often the PC checks for input.
When buying a gaming monitor, keyboard, or controller, check reviews for measured input lag figures. Look for displays with low response times (1ms to 5ms is typical for gaming models). Wired connections reduce latency. If you play competitive games, input lag should be a real consideration in your purchase, not an afterthought.