Laughter is the biological reaction of humans to moments or occasions of perceived humor: an outward expression of amusement. Laughter can be subcategorised into various groupings depending upon the extent and pitch of the laughter: giggles, chortles, chuckles, hoots, cackles, sniggers and guffaws are all types of laughter. Smiling is a mild silent form of laughing. Some studies indicate that laughter differs depending upon the gender of the laughing person: women tend to laugh in a more "sing-song" way, while men more often grunt or snort. Babies start to laugh at about four months of age. The relaxation of tension we feel after laughing may help inhibit the fight-or-flight response, making laughter a behavioral sign of trust in one's companions. Laughter is contagious; often laughter alone will provoke laughter from others.
Laughter is an explosive sound. To laugh is to make sounds that show mirth, joy, or scorn. (Webster’s New Encyclopedic Dictionary. p 566. NY: Black Dog & Leventhal Publishers Inc., 1993.)
Laughter is a form of celebration: of the unusual, irregular, indecorous, illogical, nonsensical, and unconventional. (Ornstein, Robert, PhD, and David Sobel, MD. p 217-219. Healthy Pleasures. MA: Addison-Wesley Publishing, 1989.)
Laughter is remarkably variable and may be better thought of as a broad class of sounds with relatively distinct subtypes, each of which may function somewhat differently in a social interaction. (Bachorowsk, Jo-Anne I, PhD, and Michael J. Owren, PhD. Laughing Matters. Psychological Science Agenda, Volume 18: No. 9, September 2004.)
People tend to laugh when there is incongruity between what actually happens and what they expected to happen--unless the outcome causes fear. (Greenfield, Susan A., Con. Ed. Brain Power. p 159-160. MA: The Ivy Press Limited, 1999.)