Definitions:

1. A lighthearted feeling of happiness and amusement. NOUN
   • She often brings mirthfulness to other with her bubbly personality and contagious laughter.
2. A mood characterized by high spirits and amusement and often accompanied by laughter. NOUN
   • The mirthfulness of the revelers was contagious, and pretty soon everyone in the bar was laughing.
3. A state of joyful exuberance. NOUN
   • At times he is taciturn and solemn, and then bubbles over with mirthfulness.

Synonyms:

• glee
• gleefulness
• hilarity
• mirth