How transform Variable is accessed in Scripts ?

My question is about the class hierarchy and API of Unity. I am curious and want to know how “transform” variable is accessed in our scripts by Unity. As all of our scripts are extended from MonoBehaviour Class and I don’t found any variable named transform in MonoBehaviour class. I want to know how this variable is accessed when we do this

using UnityEngine;
using System.Collections;

public class example : MonoBehaviour 
{    

  void Update() 
  { 
    transform.Translate(Vector3.forward * Time.deltaTime);
  }
}

Does anyone from where this transform variables comes ?

transform is a property of MonoBehaviour (not a variable).

Transform is a property of GameObject