What Is Joy?
joy [< French < Latin gaudere (to rejoice)] intense, ecstatic, or exultant happiness or pleasure. The biblical Greek term from Paul's spiritual 'orchard' in Galatians is chara: joy, cheer, gladness, or celebration. Joy runs deeper than mere 'pleasure', is more enduring than mere 'fun', and is more intense and thrilling than mere 'happiness'. It jumps up and shouts! Especially in a spiritual context, joy runs deep into the core of us, and radiates throughout. Joy is the response of something deep in the soul to someone (such as God, or a loved one) or something (such as being set free) supremely, even overpoweringly, wonderful.
Joy In the Bible
The Bible often defines by way of description rather than a "this is what it means" approach. So it is with 'joy'. In the Bible, there are many portraits of joy. One of the truest joys is in Psalm 113:9, the barren woman who becomes a mother. Isaiah 51 describes joy as a crown, and speaks of a future day when those whom the Lord ransoms will be overtaken by joy over it (v. 11b). In 1 Peter 1:8, loving and believing in Christ gives people the inexpressible joy of our rescue by God. In 1 John 1:4, John says that writing about the fellowship with God and believers to believers makes joy "complete". For me, one of the most joyous things of life is singing; the prophet understood this in Isaiah 52:9, with "songs of joy" over Jerusalem's return. A return to health also gives joy, and so Philip caused much joy in Samaria by way of healings. And joy under fire, overflowing into generosity, was the hallmark of the Macedonian churches, something Paul treasured. The poverty and trust of the Macedonians fits well with what Habakkuk wrote in a psalm about when Judah was being conquered (Habakkuk 3, esp. v.17-18): though there is nothing left, there is joy in knowing Who will save him. This passage also makes it clear that there is no shortage in the supply of joy; it can arise in any place and under any situation.
The joy of God's kingdom is the subject of the parable of the Hidden Treasure. In a section often used in liturgies, Psalm 51:10-12 is a prayer to God, in part, for the presence of the Spirit, and for joy to be restored to the pray-er of the psalm. God takes special joy in the person who turns away from evil and turns to God. This is apparent in the parables of the Lost Sheep and the Lost Coin; these are immediately followed by the parable of the lost son, with the father's joyous mad dash to get to his son upon seeing the son far down the road on the way home. There will be unabashed joy when the Kingdom comes in full, when all injustice is ended, when we finally see things as God made them to be. Joy, when the Lord puts an exclamation point onto His work.
There is also the joy of fools, actually a form of 'fun', rejoicing over the wrong things (Matthew 13:20, Proverbs 15:21, Revelation 18:7; also Isaiah 8:6, and in numerous OT passages where Israel's enemies are pictured as either celebrating or gloating over Israel's fall).
All this doesn't define joy, it sets its context, or gives cause for it.
Charismatics often speak of not letting anyone or any situation 'steal their joy'. Much easier said than done. Many people, whether by politics or institutions or finances or personal loss, get beaten up pretty badly by life. It can't be pretended away. It does no good not to acknowledge that reality. God did not promise that this wouldn't happen. But God puts little love notes into our lives, notes that are often so small that we miss them unless we're paying close attention. Possibilities are still there. Hope is still there. For all, not just for me. God reminds us there is cause for joy - even if perhaps not in this moment, there are more moments to come.
Joy in Music
All sorts of people find joy in music, but especially Christians. Not only do all sorts of existing styles used for worship or other self-expression, but Christians have created entirely new forms of music, such as black gospel or Gregorian chant. Whether it's "Joy To the World" each Christmas, or Beethoven's "Ode to Joy", or a multitude of songs about there being no greater joy than the tears of joy when experiencing the Savior for the first time, Christians sing of all sorts of ecstasies - as well as all other parts of life, even the ones often ignored when in church. Joy is the most essential response in our relationship with God, so much so that it cannot be contained within, it must be expressed.