A very common input device in computer games. It offers a more direct form of movement compared to directional input.
A mouse adds the freedom of a direct point-and-click interface. It typically has two clicking buttons, and most modern mice have a scroll wheel. It is a kind of input.
A mouse offers a very strong difference compared to button-based input, as it can map movement towards a position, rather than towards a direction. As its usage in conjunction with a keyboard is commonplace, and as the mouse pointer can also be tracked separately from the player's character, it can also be used as an entirely separate system from movement, such as aiming.
If your question deals with mouse-based inputs, instead of general movement inputs, this tag is appropriate to use. Using the touchpad or other equivalent on a laptop which mimics the behavior of a connected mouse will also qualify. Questions about scroll wheels, which are found on most modern mice, fall under this category as well.