Why is boiled water called tea? Where else is it called?

That's what our family calls it. We all mean whether to have tea or not. This tea doesn't mean tea. Water seems to be something to drink. Boiling water is called tea. The sugar water brewed by tea leaves is also called sugar tea. Tea brewed with tea is called tea. Here, this tea refers to something to drink. I understand that white water or something brewed with white water is called tea. Interestingly, some people call the thermos a teapot. I think this tea is drinkable.