Jack and Avery's daughter Elizabeth Donaghy is nicknamed Liddy.
Liddy is the daughter of Jack Donaghy, played by Alec Baldwin, and television reporter Avery Jessup, played by Elizabeth Banks. Her full name is Elizabeth Donaghy, connecting her to both her parents and to Liz Lemon, the show's central character.
The nickname is also a small character joke: “Liddy” echoes “Liz,” and the series states that Elizabeth was named after Liz Lemon. Jack and Avery's relationship develops during the later seasons, and their marriage is tied to Avery's pregnancy. They eventually divorce after Avery returns from being held captive in North Korea.
A common mix-up is to assume that Liddy is Liz Lemon's child because of the name connection. Liz is not Liddy's mother; she is Jack's coworker and close friend. The character is primarily part of Jack's family storyline rather than a regular on-screen member of the TGS workplace.