Laughter is an audible expression or the appearance of happiness, or an inward feeling of joy (laughing on the inside). It may ensue (as a phyiological reaction) from jokes, tickling or other stimuli. It is in most cases a very pleasant sensation.
Laughter is found among various animals, as well as in humans. Among the human species, 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.