问题
The official documentation for the Unity Engine doesn't contain this, and I'm not far enough into my math/physics studies to have come across a Quaternion, but I understand it has to do with rotation. What I don't understand is why Quaternions have four variables, w,x,y,z, when there are only three axis of rotation in Unity.
回答1:
"A quaternion is basically an axis in 3D space with a angle of rotation around the axis. Four values make up a quaternion, namely x, y, z and w. Three of the values are used to represent the axis in vector format, and the forth value would be the angle of rotation around the axis."
http://www.real3dtutorials.com/tut00011.php
So you could think of it as the rotation of the rotation, in simple terms!
Like Hellium noted in the comments below; Unity recommends that you do not fiddle with Quaternions directly if don't know exactly what you're doing. Like Hellium also points out, whatever you want to accomplish, you probably want to use the static methods of the Quaternion class. They are very useful and easy to use and can accomplish most things you want to do with rotations.
来源：https://stackoverflow.com/questions/42285413/why-do-quaternions-have-four-variables