The title is a German word meaning "compulsion to move," and refers to a situation in Chess wherein one player is put at a disadvantage because they must make a move when they would prefer to pass and not move. The fact that the player is compelled to move means that their position will become significantly weaker. A player is said to be "in zugzwang" when any possible move will worsen their position.
Reid mentions that when he and Maeve talk, they address each other by pseudonyms. Reid's is "Dr. Joseph Bell," Sir Arthur Conan Doyle's inspiration for Sherlock Holmes. In God Complex (2012), Reid calls Maeve and, while waiting for her callback, reads the Holmes novel "The Sign of Four."
Maeve is a geneticist. Garcia states she was on sabbatical from Mendel University. Gregor Mendel is the father of modern genetics.