Beginning with a dictionary definition is the tactic of the cowardly writer, and yet, here we go: According to an aggregation of medical dictionaries, "tickling" denotes "a peculiar itching or tingling sensation caused by excitation of surface nerves, as of the skin by light stroking," occasionally accompanied by "involuntary laughter."
That gives us a good foundation from which to build, but still doesn't explain the central question: What the hell's up with tickling?
Sure, the question sounds like a bad comedy set, but from an anatomical, biological -- hell, even social -- standpoint, why are humans ticklish? Could it have been crucial to our evolution, and remained, like the appendix, a part of who we are even though it serves no purpose? Does it serve a function unnoticed or long forgotten?