Peppermint (Mentha x piperita)
Peppermint tea is a tisane (infusion) of peppermint, Mentha piperita. It is sometimes called mint tea. It is naturally caffeine-free. A tea made from blending peppermint and spearmint (Mentha spicata) leaves is referred to as doublemint tea.
It does not contain any Camellia sinensis and thus does not become bitter if brewed too long. Some people prefer to add a small amount of sugar.
For a more comprehensive treatment, see the article on 
Wikipedia.