The rotation of the transform in world space stored as a Quaternion.

Unity stores rotations as Quaternions internally. To rotate an object, use Transform.Rotate. Use Transform.eulerAngles for setting the rotation as euler angles.

	// Reset the world rotation
	transform.rotation = Quaternion.identity;

using UnityEngine;
using System.Collections;

public class Example : MonoBehaviour {
    void Example() {
        transform.rotation = Quaternion.identity;
    }
}

import UnityEngine
import System.Collections

public class Example(MonoBehaviour):

	def Example() as void:
		transform.rotation = Quaternion.identity

Another example:

	// Smoothly tilts a transform towards a target rotation.
	var smooth = 2.0;
	var tiltAngle = 30.0;
	function Update () {
		var tiltAroundZ = Input.GetAxis("Horizontal") * tiltAngle;
		var tiltAroundX = Input.GetAxis("Vertical") * tiltAngle;
		var target = Quaternion.Euler (tiltAroundX, 0, tiltAroundZ);
		// Dampen towards the target rotation
		transform.rotation = Quaternion.Slerp(transform.rotation, target,
		                               Time.deltaTime * smooth);;
	}

using UnityEngine;
using System.Collections;

public class Example : MonoBehaviour {
    public float smooth = 2.0F;
    public float tiltAngle = 30.0F;
    void Update() {
        float tiltAroundZ = Input.GetAxis("Horizontal") * tiltAngle;
        float tiltAroundX = Input.GetAxis("Vertical") * tiltAngle;
        Quaternion target = Quaternion.Euler(tiltAroundX, 0, tiltAroundZ);
        transform.rotation = Quaternion.Slerp(transform.rotation, target, Time.deltaTime * smooth);
    }
}

import UnityEngine
import System.Collections

public class Example(MonoBehaviour):

	public smooth as float = 2.0F

	public tiltAngle as float = 30.0F

	def Update() as void:
		tiltAroundZ as float = (Input.GetAxis('Horizontal') * tiltAngle)
		tiltAroundX as float = (Input.GetAxis('Vertical') * tiltAngle)
		target as Quaternion = Quaternion.Euler(tiltAroundX, 0, tiltAroundZ)
		transform.rotation = Quaternion.Slerp(transform.rotation, target, (Time.deltaTime * smooth))