Tea is a plant. It's one plant but comes in many varieties. I like to compare it to grapes because with grapes you can make wine. So just like grapes there is tea. The scientific name is called Camellia Sinensis. Depending on where it's grown it can produce a variety of flavors as well. It really is an amazing plant. It has a lot of great qualities. Especially because of the amino acid it carries. L-Theanine. It breaks the blood brain barrier and gives you a quick sense of calm.
Peppermint is not a tea. Rooibos is not a tea. Chamomile is not a tea. If it is not produced by a camellia sinensis then it's an herb. If you brew it like a tea it is called a tisane. If no tea is contained in your steeping of whatever plant you put in the hot water then it's a tisane. However, lemon or honey added to hot water is not considered a tisane. It's called lemon-water. Or water with honey in it.
Tea can come as white, green, oolong, black, or pu-ehr. Those are the ones common in the United States currently. There are other varieties that haven't quite caught on but they are out there. All tea is the same except for the way it's prepared. Green and white teas are preserved whereas oolong and black teas have been oxidized. There are different effects as well as caffiene levels in each type of tea based on its processing. Green and white are better overall but most Americans prefer the black teas. If you are drinking tea to keep optimal health functions then stick to white and green. This does not mean it's a cure-all but great at preventing the worst of symptoms and assists with recovery quicker. That is if you have tea in your system before you become ill. Oolong is great for digestion.