That's a good one, UnderPar!
I'm no theologian, so can't guarantee that my answer is the theologically correct one. I imagine that heaven means different things to different Christians.
Basically Heaven is where God is - not so much geographically, but in essence, in presence. The idea of experiencing "heaven on earth" is very real for me.
In the Bible "the heavens" can be the sky (i.e. the atmosphere), or all of space (as we have come to know it) or simple where God is. So context is important when interpreting a specific text in the Bible.
The New Testament also uses the terms "kingdom of God" and "kingdom of heaven." In his famous prayer Jesus requests that "your (God) kingdom come, your will be done on earth as it is in heaven." So one feature of heaven is that it is where God's will is done. And Jesus obviously seems to be indicating that Christians should not be focusing on "going to heaven when we die" but on bringing heaven to earth in the here and now.
Jesus also said "the kingdom is near (in time and place)" and "the kingdom is in you."
In the kingdom of heaven Love reigns, the Truth is supreme, there is true justice, people become all they were meant to be, they are free to serve one another with no ulterior motives, there is freedom to accept service with grace and without suspicion of demands of reprocity, there is genuine delight in others, there are no power games. Jesus himself provides the role model, the focus, the rationale, the goal, the purpose and the means. And that means that there is much more than what I have described.
And if Jesus is to be beliieved, it is possible to get a real glimpse of heaven materialised here. When I look around me at certain people, I can already see evidence of heaven on earth.
