Any situation where one group is forced to go along with what another group wants is tyranny, regardless of the sizes of the groups. When the first group is large and the second group is small you have an oligopoly or dictatorship. When the reverse is true you have tyranny of the majority, which isn't much of an improvement.
The democratic ideal is people coming together voluntarily to work toward their common interests, with no one being forced to participate against their will. The right to secede is key—it's not a legitimate democracy if you aren't allowed to leave.
I don't agree. Ideally, everyone could make their own decisions for their own so they aren't forced to go along with others. In that case there isn't even a need for democracy. But frequently, that is not possible – what you call "tyranny" I consider unavoidable and democracy the best way to deal with it.
Majority rule is inherently better than rule by any specific group in two respects: One, more people benefit from it. Even if you have two blocks who always vote together, it's better if 60% benefit at the expense of 40% than vice versa. Two, that doesn't actually happen: Sometimes you are on the winning side, sometimes on the losing one so it somewhat (not perfectly) cancels out.
And democracy isn't just majority rule. Separation of powers and a catalog of fundamental rights are also important to ensure everyone's interests are considered when making a collective decision. So is a culture of just doing so, when voting and in general.
Sure you could count every instance of someone not getting their will, which is unavoidable when decisions affect more than one person, as tyranny. But I don't think that's a very useful definition.