...Hm, I dunno. Basically, life essence that all creatures have. Humans can learn how to specifically manipulate that aura, which is called Nen. It's kind of through like—martial arts, I guess. When you do stuff like the little electricity spark I did, that's a form of Nen called "Hatsu"!

[Killua is sure that sounds insane. Hunters aren't allowed to explain Nen to ordinary people in his own world, so he understands from the jump how crazy that might sound...]