Application.Quit

Description

Quits the player application.

Quit is ignored in the editor. IMPORTANT: In most cases termination of application under iOS should be left at the user discretion. Consult Apple Technical Page qa1561 for further details.

// Quits the player when the user hits escape
function Update () {
    if (Input.GetKey ("escape")) {
        Application.Quit();
    }
}

using UnityEngine;
using System.Collections;

public class ExampleClass : MonoBehaviour {
    void Update() {
        if (Input.GetKey("escape"))
            Application.Quit();
        
    }
}