Shorthand for writing Vector3(0, -1, 0).

   transform.position += Vector3.down * Time.deltaTime;

using UnityEngine;
using System.Collections;

public class Example : MonoBehaviour {
    void Example() {
        transform.position += Vector3.down * Time.deltaTime;
    }
}

import UnityEngine
import System.Collections

public class Example(MonoBehaviour):

	def Example() as void:
		transform.position += (Vector3.down * Time.deltaTime)