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)