What are the authentic traditional ingredients for Naan bread?

Naan traditionally is plain flat bread made using bread flour, Yeast, salt and water. Its cooked in tandoor.

Salt could be optional if you are having naan with a curry. (Cause curry usually has salt and the bread might not need it).

Variations like milk or yogurt is used instead of water to make dough soft and fluffy. This would change the texture and flavor of the bread.

It is possible to use baking powder instead of yeast, because they both act as a leavening agent. Don't use them together.

IMHO eggs are not part of the traditional naan ingredients.

Naan is a catch-all term for flat, leavened bread throughout the asian subcontinent. There are many, many variations which have developed depending on what ingredients were typically to hand in the different regions. So there are many equally traditional and authentic variations. In other words the answer is that there is no answer, try a few and find out what works for you.

About the recipe:

Naan is traditionally made with yogurt to leaven it. It would have to sit for at least 4 hrs, or up to 12, depending on how warm it is. Yeast was not traditionally used in india (though is now), and baking soda is also a newer introduction.

Milk or water can be used interchangeably. Milk will make it a bit softer. This is just preference.

Egg is not necessary. Many people in India do not eat egg, so the default recipe would be without egg.

About how to cook them:

People at home rarely make naan -- it is something you would have out or at a wedding or somewhere with a tandoor. This is why there are so many variations -- it is just people adapting as best they can.

If you have a pizza or baking stone, you can get a really good naan by preheating the stone at your highest oven temperature, and just putting the naan onto it to cook -- this simulates a tandoor.

Otherwise, I have found the best thing is a griddle, again as hot as you can get it.

It works best if you keep the dough soft and roll it out with a little oil as thin as you can get it (it will "shrink back", so make it bigger than you actually want it).

General baking tips:

Baking soda will kill the yeast and yogurt bacteria. You can add it as "insurance", but do this at the end after fermentation. They also don't like salt (but salt will make your bread taste a LOT better), so also add that at the end.

I have not tried mixing yeast and yogurt, but it doesn't sound like a great idea. Maybe it is ok if you either reduce the amount of yogurt a lot, or add it again at the end after fermentation.

Maida, awesome, I get it from this importer in town: http://en.wikipedia.org/wiki/Maida_flour

Its almost the worst flour for you, but Naan is a "fast-food", so you can forgive the lack of nutrition for the flavour.

The rest is covered in the accepted answer.

There are many ways to cook naan depending upon the region or area, all are authentic but as far as the addition of egg in naan is irrelevant. People use yogurt, ghee (clarified butter) as well as milk to make soft naan. Yeast and dough starter both are used interchangeably by most of the naan maker.

Breaking down the ingredients:

1. Flour: always use strong/bread > 12% protein if you want strong gluten network
2. Milk: always use whole milk: tenderizes the Naan + flavor
3. Yogurt similar effects as Milk but also contains bacteria which aids in fermentation
4. Water: water is water: either use powdered milk with water or just milk preferably whole
5. Baking Powder/Soda: never use unless you are in a rush, always use yeast for better taste, as Baking Powder does not contribute to fermentation only leavening
6. Yeast: leaven the dough and ferments the flour,it breaks down the sugars, thus removing the unpleasant taste of flour and adds a good taste to the dough, for best results ferment from 24 hours up to 3 days. or use a Poolish preferment ahead of time 8-18 hours depending on temperature and yeast added (50% Flour 50% Water 0.1% yeast)
7. Eggs : gives leavening, color, texture, and flavor
8. Salt: Taste and clolor

As many have said Naan has many variations, no size fits all. after reading the break down of the ingredients above, I would recommend trying different combinations to get the taste you really like.

Pro Tip: The Tandoor gets really hot about 900F/482C, when baking in a home oven it can reach up to 560F most oven at 500F, so the cooking time will be longer, to make the end result close, I would recommend adding more water (70-80% hydration), as more will be evaporated during the extra cooking time. The more hydration the more sticky and harder to work with the dough would be.