Has anyone ever told you, "um actually... mint tea isn't really tea"? Of course, they're being pretentious, despite being technically correct (emphasis on technically). Very simply, tea is a beverage made from the leaves of Camellia sinensis, the tea plant. This is a bit of an oversimplification as there are other Camellia species that also make what is considered "tea", but for our purposes, Camellia sinensis is the plant which makes tea. Any leaf beverage made from other plants is not tea, it is a tisane.
NOW, that being said, because nearly everyone in the English speaking world considers tea to really be any steeped leaf beverage, if people correct you when you say "mint tea", they're being a bit ignorant of how the word tea is colloquially applied. But if you find the distinction interesting, I'll break it down one more time.
Tea: Beverage made from the steeped leaves of the tea plant (typically Camellia sinensis, but not strictly limited to this one species - other Camellias CAN be considered to make tea).
Tisane: Any beverage made of steeped leaves that is not defined under another umbrella (i.e. yerba matte, tea).