Tickling is the act of touching a part of the body so as to cause involuntary twitching movements or laughter. The word "tickle" evolved from the Middle English tikelen, perhaps frequentative of ticken, to touch lightly. The idiom tickled pink means to be pleased or delighted.
In 1897 psychologists G. Stanley Hall and Arthur Allin described a "tickle" as two different types of phenomena. One type caused by very light movement across the skin. This type of tickle, called knismesis, generally does not produce laughter and is sometimes accompanied by an itching sensation.
Another type of tickle is the laughter inducing, "heavy" tickle, produced by repeatedly applying pressure to "ticklish" areas, and is known as gargalesis. Such sensations can be pleasurable or exciting, but are sometimes considered highly unpleasant, particularly in the case of relentless heavy tickling.