There a lot of definitions of what is a Tea. My version is the following: In sensu stricto, tea is an infusion of the Camellia sinensis leaves and leaf buds. Camellia sinensis is the scientific name of the plant that produces tea. Oolong, green, white and black teas are examples of these teas.

In sensu amplo, the word tea is an infusion of seeds, roots, flowers, bark, fruits, leaves, spices and its combinations. It is common to call the non Camellia sinensis teas “herbal teas” or “tisanes”. The “tisanes” are typically non-caffeinated brews.

Usually when we say the single word tea, we are talking about regular black tea. All other teas have two words to describe the type of tea, e.g., Masala Tea, Honey Dew Tea, Ginger Tea, Lemongrass Tea, Mint Tea, etc.
If you are in a foreign and want to order a tea, one of the three words (te, chay or cha) will get you a tea. What is called commonly tea may have a different way to brew and serve from country to country, but it will be a tea.

The name Camellia is the Latinized surname of the botanist Georg Kamel, and sinensis means Chinese in latin.

Interesting message in a teacup on the Alitalia flight from Rome to Verona last week.

In the Mercado Publico (Public Market) in Porto Alegre, Brazil, teas are sold in small packages. They are called Chá, here the teas are labelled with the common name and some with the scientific name too.

At the Spicy Market in Istanbul, Turkey the teas are called Çay and are sold bulk together with the spices