PrimitiveType.Plane  

// Creates a plane primitive
function Start () {
var plane : GameObject = GameObject.CreatePrimitive(PrimitiveType.Plane);
}

using UnityEngine;
using System.Collections;

public class example : MonoBehaviour {
void Start() {
GameObject plane = GameObject.CreatePrimitive(PrimitiveType.Plane);
}
}

import UnityEngine
import System.Collections

class example(MonoBehaviour):

def Start():
plane as GameObject = GameObject.CreatePrimitive(PrimitiveType.Plane)