> The definition of "conservative" means "not-extreme."
No, it really doesn't. I mean, yes, that's a definition of "conservative" in common language, but it has never been the definition of "conservative" as a label of political ideology; like many words, "conservative" means different things in different contexts.
Saying, in a discussion of political ideologies, that "conservative" means "not extreme" is like saying in a discussion of programming paradigms that "functional" means "designed to be practical and useful, rather than attractive". That is absolutely a definition of the word, but not the one relevant to the context at hand.
As a political ideology label, "conservative" was defined in reaction and opposition to liberalism and the outward distribution of power away from traditional institutional, hereditary, economic, and religious elites that it represented, and refers to the defense of the privilege and power of such elites and the traditions that sustain and emanate from them within the politico-economic system.
Now, over time since then, as there has been more progress made by liberal and other newer forces against the elites of the time that distinction arose, and even sometimes against the newer elites that arose because of early liberal successes like the bourgeoisie who displaced the feudal aristocracy as the ruling class in the capitalist world, to see their own power somewhat eroded in the transition to mixed economies, there has come to be a distinction sometimes made between plain "conservative" being the a sort of mostly-status-quo-ist defense of current elites that mostly opposes weakening their power and favors very modest steps to shore it up, versus reactionaries that favor more extreme action either to deeply retrench the power of status quo elites or to actually wind back power to past-but-currently-displaced elites -- but even in that terminology reactionaries do not stand in opposition to conservatism but simply stand further out in the same direction. There is a good argumen that the GOP was transitioned over time from plain conservative to outright reactionary, but that's not a change in direction.