Application.Quit

Description

Quits the player application.

Shut the running application down. Quit is ignored in the editor.

Note: In most cases termination of application under iOS should be left at the user discretion. Consult Apple Technical Page qa1561 for further details.

using UnityEngine;
using System.Collections;

// Quits the player when the user hits escape

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