Knismesis is one of two types of tickling and is a scientific term first used in 1897 by G. Stanley Hall and Arthur Allin in The American Journal of Psychology. Knismesis describes the light, "barely being touched" stimulation that makes the recipient want to itch, not laugh -- almost as though bugs were creepily crawling on the skin.
The other form of tickling is called gargalesis and occurs when the tickler focuses on the recipient's sensitive areas with a more aggressive touch, perhaps repeatedly, in order to elicit a laughing response. This "heavy tickle" is usually associated with play, quite often with children.
Other facts about tickling: