What does this mean? If you go out to have lunch or dinner with a guy (he has a girlfriend and she probably doesn't know it), does it consider as a date or a normal hangout? Does this mean that he cheat? and the girl who go with him cheat?

what?????

Depends on the setting and topic of conversation.

I grew up with a LOT of friends whom were girls, and I can honestly say, I went to dinner with a couple, and it's no different then best buddies hanging out with a couple of beers . . .

Here's the key, communicate to your wife or girlfriend. Dinner can be in a loose and comfortable setting where you can still hear each other but it's not intimate. Sharing a meal with an old or current friend of the opposite sex shouldn't feel weird . . . It gets weird when it's a secret from your spouse. If your spouse is uncomfortable, then invite them along . . . With the understanding that there WILL be times when you are talking about jokes or experiences that only relate to you and your friend. 

Bottom line, we are more evolved then an animal just out to propagate the species, we are, at our core, a social animal . . .

My concern is that the girlfriend doesn't know. I couldn't care less if my husband has a meal with a friend regardless of their gender. Well, actually let me clarify that. There is only on woman from his past I would care if he met. She has proven herself time and again to be a liar and a user and I don't want her in our lives ever again. Other than her my husband can meet whomever.

If he doesn't tell his girlfriend that he's having dinner or lunch with another woman .. He's hiding something.  If it was "innocent", he would tell her.  But if the woman he's having a meal with knows he has a girlfriend and still goes .. She's no better than he is.

Depends where he is taking you for a dinner. If it is a formal dinner then it isn't just a friendly hang-out. Friendly date is eating in a fast food restaurant.

Only you and he can really answer this question.

Examine your motivations and the possibility of unacceptable or unintended consequences and the resultant effects on all the persons who may be involved, including those only potentially affected.