Level Up (A5E) No explanation for what a Reaction is in the AG

[lichmaster]

Right, but you can't really make a list of possible reactions very effectively as some reactions are spells, some are maneuvers, some are attacks, etc. A reaction can be anything, hence why it is not specifically defined.

Well it could simply state that there may be some situations occurring outside of your turn where you can react. And it could clarify how it's supposed to work (trigger, etc). It could also make the example of the attack of opportunity

 

[lichmaster]

I wonder if it is also missing from the 5.1 SRD and that is why.

Well, they can't include in LU what's not in SRD if it's specific to WoTC material, like Mind Flayers etc. Rules are not included in this kind of reasoning AFAIK.

 

[Corinnguard]

The definitions for Action, Bonus Action and Reaction aren't in the A5eSRD. It covers what's already in the Adventurers' Guide.