For the Christian, heaven is where Jesus is. We do not need to speculate on what heaven will be like. It is enough to know that we will be for ever with Him.
But the best definition of it is to say that heaven is that state where we will always be with Jesus, and where nothing will separate us from Him any more.
When you put your total faith in God, no matter what happens, to a person who's a true believer, if you die, you know you're going to heaven to be with God.