Middle mouse button (or third button).

Use this to detect middle mouse button presses. The “2” mouse button is the third button on the user’s mouse, which is usually the middle mouse button (often a clickable scroll wheel). Unity defines this as the "2" mouse button, as the mouse button numbering begins at 0.

using UnityEngine;

public class Example : MonoBehaviour { void Update() { //Detect if the middle mouse button is pressed if (Input.GetKey(KeyCode.Mouse2)) { Debug.Log("Mouse 2 "); } } }