In order for something to be considered a true "tea," the plant that is used must come from the tea plant, camellia sinensis. Therefore, the delicious herbal delight we all drink, cannot be called true tea.
To put it simply...a tea is "only a true tea" if it actually contains tea plant leaves. And thus...this is why oolong, white, green and black are considered "true teas," as their leaves come from the actual tea plant camellia sinensis. By contrast, rooibos and herbal teas do NOT contain leaves from the tea plant.
Confused? Read What Tea Is and What Tea is Not for more information.
Thus, with this out of the way, we can basically define a herbal tea as just about anything that gets added to boiling water. And by anything...I do mean ANYTHING. From lemons, to orange peel, to whole strawberries, to peppermint leaves, to candy cane pieces...and well, you get the idea?