Jack Bing is one of Monica and Chandler's two children. Monica and Chandler adopted Jack and his twin sister Erica from a girl, also named Erica, living in Ohio.

Jack and Erica were born in the series finale. Monica and Chandler were unaware at the time of their birth that they would be receiving twins and Monica had to calm Chandler down at first. Upon seeing Jack for the first time, Chandler commented that he had Monica's eyes (despite acknowledging that it was impossible for this to actually be the case).
The Twins

Jack (held by Monica)

Jack was named after his adopted grandfather. Phoebe commented that his full name sounded like a 1940's reporter.