Wistful (adjective)

1. Having a feeling of longing or desire tinged with melancholy.
2. Having a feeling of longing for a person, place, or thing that is not present.
3. Having a feeling of regret or sadness for what might have been.

Origin:

Middle english wistfull, from old english wistful; akin to old high german wuostful wistful.

Examples:

1. She had a wistful look on her face as she thought of her childhood home.
2. He had a wistful smile as he thought of his old friends.
3. She gazed at the picture with a wistful expression.
4. The wistful melody filled the room.
5. He had a wistful longing for the days of his youth.