Pensive (adjective)

1. Appearing or characterized by deep, often melancholy, thought.
2. Reflective; contemplative; meditative.

Origin:

From old french pensif, from latin pēnsīvus ("thoughtful"), from pēnsus ("thought"), past participle of pendere ("to weigh, to consider").

Examples:

1. She looked pensive as she stared off into the distance.
2. The pensive expression on his face made it clear that he was deep in thought.
3. His pensive mood was evident in his music.
4. The pensive tone of the poem.
5. He was lost in pensive thoughts.