Point of Divergence or POD, also known as point of departure, break-point, or several other synonyms, is a part of alternate history fiction. It is the moment when the fictional world's history stops following the same course of events as our timeline.
Many alternate history stories use an important battle as a Point of Divergence. In the Southern Victory series, the POD occurs on September 10, 1862, when Order 191 lost by a Confederate courier are immediately recovered by Confederate troops. In our timeline, the orders were discovered by Union forces. The consequence of that change is that the Confederate States win the American Civil War and go on to become an independent nation from 1862 until 1944.
A Point of Divergence can also be a natural event not related to the acts of human beings, and might be in the distant past. For example, in the Atlantis series the POD is related to tectonic forces working differently than in OTL, 85 million years in the past, causing a subcontinent to break off from North America and drift into the mid-Atlantic becoming what would become Atlantis; in A World of Difference the POD is related to planet-forming forces creating, in the fourth orbit outwards from the Sun, a planet substantially different from our Mars.
Some stories have an initial POD and a relevant POD. In Joe Steele, which focuses on an alternate American history, the initial POD comes in 1878 when the Djugashvili family emigrate from Russia to America. Their son Iosif thus grows up as an American named Joe Steele rather than the Soviet figure Joseph Stalin he became in OTL. However, this does not affect American history until the relevant POD in 1932, which happens when Steele uses brutal tactics to secure the Democratic nomination for the presidential election. In A World of Difference, the initial POD comes millions of years ago when the development of Mars is altered, as noted above, but the relevant POD comes only in 1976 when NASA space probes discover Minervans.
Occasionally Turtledove writes a story without a specific POD in mind beyond a generic reversal of a certain historical period. He has stated that in The Gladiator, he simply wanted to depict a world where the Soviet Union outpaced the United States at every step of the Cold War. The novel does not point to any one event as the POD, simply stating that the divergence happened in the mid 20th century, but the first noticeable difference from OTL occurs in 1962.
Sometimes Turtledove specifies the date of the POD within the story, e.g. "Must and Shall" and "Ready for the Fatherland" pinpoint their PODs as July 12, 1864 and February 19, 1943, respectively. At other times, the POD is only referenced generally, e.g. most of his Nazis-win-World War II stories have PODs vague enough to be either in 1940 or 1941, and The Two Georges puts its POD anywhere between 1763 and 1768. In these and other cases, the above list uses the most reasonable estimate.
A * indicates that the relevant POD is not the initial POD.