Laughter is an audible expression or appearance of happiness, or an inward feeling of joy (laughing on the inside). It may ensue (as a physiological reaction) from jokes, tickling, and other stimuli. Laughter is found among various animals, as well as in humans. Among the human race, it is a part of human behavior regulated by the brain, helping humans clarify their intentions in social interaction and providing an emotional context to conversations. Laughter is used as a signal for being part of a group — it signals acceptance and positive interactions with others.

Laughter is sometimes seemingly contagious, and the laughter of one person can itself provoke laughter from others as a positive feedback. This may account in part for the popularity of laugh tracks in situation comedy television shows. Modern neurophysiology states that laughter is linked with the activation of the ventromedial prefrontal cortex, which produces endorphins after a rewarding activity. According to Cindie Leonard, Solid scientific research demonstrates that laughter offers the following benefits:

• Lowering blood pressure
• Strengthening cardiovascular functions
• Reducing stress hormones
• Improving circulation
• Increasing muscle flexion
• Oxygenating the body by boosting the respiratory system
• Boosting immune function by raising levels of infection-fighting T-cells, disease-fighting proteins called Gamma interferon and B-cells, which produce disease-destroying antibodies
• Triggering the release of endorphins, the body's natural painkillers producing a general sense of well-being