What does E.N.D mean?

all we know about E.N.D. right now is that he is a 火災, 火 demon created によって Zeref. He is the strongest one and he is the leader of Tartarus, he is so strong that even Igneel was not able to defeat him. Right now he is celled in his book and Tartarus are trying to erase magic to be able to revive him.
(E.N.D. is in that book)

E.N.D is a book of destruction that created によって Zeref.
We all still don't know what the meaning of "Who will awaken the E.N.D." because E.N.D is Natsu. Let's wait for the future to answer that. But the real name of Natsu is actually;

E therious
N atsu
D ragneel

So when we shortened the name.. it results " E.N.D"
E.N.D is ' Etherious Natsu Dragneel '. And Btw, if あなた haven't read the マンガ then read it hurry, tartaros arc is the best arc ever. All ドラゴン of the dragon slayers will fright against 'face' and igneel, will fight arconologia. Eventho unfortunately igneel will die. Sorry for spoilers :P

Just go to tumblr and read the E.N.D theory !

I Believe Natsu is Zeref's Brother they had a family 400 years 前 Natsu was young. Then their parents and Natsu died によって ドラゴン (I think they died によって ドラゴン but Natsu and his parents died 400 years ago). But Zeref Survived. Zeref found his little brother's body (Natsu) he revived Natsu and made him in the 本 of Zeref he called him Etherious Natsu Dragneel the reason he called Natsu Etherious because he was now in the 本 of Zeref anyone from the 本 of Zeref is a demon and is called Etherious so Natsu is a Demon and he is E.N.D (Etherious Natsu Dragneel). E.N.D was meant to kill Zeref, Zeref created Natsu because he wanted to die but I don't know why? (My answer may have spelling mistakes it's because I rushed it).

E.N.D is stand for Etherious Natsu Dragneel that Zeref created

Also, so far the only people who know that Natsu is E.N.D. (that we know of) is Zeref, and Kind Animus from the Fairy Tail: Dragon cry movie. E.N.D. stands for Eterious Natsu Dragneel.