These days, it's "an open smile, on a friendly shore." (The Looooove Boat, la-la-la.)
In all seriousness ..... doesn't a sincere smile just speak volumes?
I suppose the best definition I've heard was a story told by my former minister:
One day she asked her little son, "What if, hundreds of years ago, you were living and playing in Nazareth ...... Do you think you could identify which child was Jesus Christ?"
The child thought for a moment, then answered decisively, "Yes!"
"But, how? How would you know?"
"Because ... of the way He makes me feel."