Why is Kingdom Hearts called Birth by Sleep?

Answer 1

*Spoiler-Heavy content*

Kingdom Hearts Birth by Sleep got its name from the conclusion of the story. When Ventus destroyed Vanitas, his heart was no longer complete. He then fell into a deep sleep. This sleep will be ended when his heart is together once again, hence the name "Birth by Sleep." We last see Ventus being woken up by Terra, indicating that he has been "born again". We don't exactly know, as of yet, what time period this event occured. Be it before or after Kingdom Hearts/Kingdom Hearts II.

This also might be an explanation to the character "Roxas", seeing how Ventus was placed in Sora's heart. This is another way that the word "birth" comes into the title.

Answer 2

After destroying Vanitas and the x-blade, Ven goes to sleep and his heart sleeps within Sora. Then Sora gains the ability to use the keyblade.