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In the definition of a natural transformation from http://ncatlab.org/nlab/show/natural+transformation

it is said that a natural transformation is $\alpha : F \Rightarrow G$ (and the definition continues)

But what does that $\Rightarrow$ mean, how is it called?

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    @Zhen: that's a very good point. In fact, I don't know where the notation comes from but it is pretty standard in homological algebra nowadays even without mention of $2$-categorical ideas. I checked the usual suspects and the first reference I've found up to now using the $\Rightarrow$ notation is Bénabou's *[Introduction to bicategories](http://dx.doi.org/10.1007/BFb0074299)*. It doesn't appear that it was used earlier by any of the *founding fathers* Eilenberg-Mac Lane, Ehresmann, Freyd, Gabriel, Grothendieck, Kan, etc.2011-08-21

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Just look in the comments, especially the one by t.b., $\Rightarrow$ is just the notion of natural transformation, nothing more general.