It would be considered assault. And it can land one in jail just as easily as if you punched a person in the snoot.
Myriads
There's a big difference between punching someone in the face and tickling them.
Physical assault will get the attention of the system: restraining orders, convictions, lawsuits, etc.
Tickling will get you absolutely none of that.
I, for one, would like to see you back up this claim with one example ... just ONE, where someone actually went to jail in the U.S. for tickling another person.
(a Google search produced one article of very questionable nature where a guy from Uganda got 5 years. No names, nothing. It was maybe 2 sentences long)
I've done it plenty of times and the worst that happened was they were pissed off.
Short of kidnapping and sexual assault, tickling isn't even on the radar when it comes to the law. Bondage against their will would probably have to be involved.
If it's consensual bondage in a play situation, all bets are off.
If it's a woman tickling a man, you'll be laughed at by anyone who'll even listen to you.
Damn ... this place is crawling with legal experts and jailhouse lawyers.