A Quad primitive.

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

using UnityEngine;
using System.Collections;

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

import UnityEngine
import System.Collections

public class Example(MonoBehaviour):

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