The green axis of the transform in world space.

	// Set's the rigidbody velocity to be 
	// along the green axis of the transform
	rigidbody.velocity = transform.up * 10;

using UnityEngine;
using System.Collections;

public class Example : MonoBehaviour {
    void Example() {
        rigidbody.velocity = transform.up * 10;
    }
}

import UnityEngine
import System.Collections

public class Example(MonoBehaviour):

	def Example() as void:
		rigidbody.velocity = (transform.up * 10)