Minecraft wiki entry for JSON_format, associated text:  conditions:
 lightning: The lightning bolt that disappeared. May also be a list of predicates that must pass in order for the trigger to activate.
All possible conditions for entities[show]
 bystander: An entity not hurt by the lightning strike but in a certain area around it. May also be a list of predicates that must pass in order for the trigger to activate.
All possible conditions for entities[show]
 player: The player that would get the advancement. May also be a list of predicates that must pass in order for the trigger to activate.
All possible conditions for entities[show]