There are romantic hugs, too, but that is not what these teenagers are talking about.

Girls embracing girls, girls embracing boys, boys embracing each other  the hug has become the favorite social greeting when teenagers meet or part these days. Teachers joke about “one hour” and “six hour” hugs, saying that students hug one another all day as if they were separated for the entire summer.

A measure of how rapidly the ritual is spreading is that some students complain of peer pressure to hug to fit in. And schools from Hillsdale, N.J., to Bend, Ore., wary in a litigious era about sexual harassment or improper touching  or citing hallway clogging and late arrivals to class  have banned hugging or imposed a three-second rule.

Parents, who grew up in a generation more likely to use the handshake, the low-five or the high-five, are often baffled by the close physical contact. “It’s a wordless custom, from what I’ve observed,” wrote Beth J. Harpaz, the mother of two boys, 11 and 16, and a parenting columnist for The Associated Press, in a new book, “13 Is the New 18.”

“And there doesn’t seem to be any other overt way in which they acknowledge knowing each other,” she continued, describing the scene at her older son’s school in Manhattan. “No hi, no smile, no wave, no high-five  just the hug. Witnessing this interaction always makes me feel like I am a tourist in a country where I do not know the customs and cannot speak the language.”

For teenagers, though, hugging is hip. And not hugging?

“If somebody were to not hug someone, to never hug anybody, people might be just a little wary of them and think they are weird or peculiar,” said Gabrielle Brown, a freshman at Fiorello H. LaGuardia High School in Manhattan.