The Butterfly Effect is the notion that small changes in conditions can cause larger ultimate changes with no easily apparent logical pattern. The name is derived from the theoretical example of the flapping of a butterfly's wings in one part of the world affecting whether or not a hurricane forms in another part.
The phrase entered alternate history literature through Ray Bradbury's short story "A Sound of Thunder". In that story, a time traveler accidentally kills a butterfly millions of years ago, causing massive social changes on his return.
Any changes in an alternate history which are possible but not necessary, are the result of the butterfly effect. In the 1632 series, a frequent result of the butterfly effect is historical figures dying sooner -- or later -- than they did in the OTL, even if it does not appear that changes in events have greatly affected their personal circumstances. For example, in the 1632 series, one Johann Georg II, Count of Solms-Baruth, did not die of plague in early 1632, as he had in the OTL. As a result, he is alive in the spring of 1633, and stubbornly contesting that he, not Don Balthasar (Baltasar) de Marradas, is really the imperial administrator in Prague. The enmity and squabbling between the two men affected the planning for Wallenstein's coup, as it interfered with the Austrians' ability to maintain firm control in Prague and Bohemia.
In much alternate history literature, another consequence of the butterfly effect is that very few, if any, people conceived very long after a "point of divergence" will exist in the alternate history, as sperm cells are very sensitive to environmental conditions. A very small change that is unnoticeable on the macro level can affect which sperm cell fertilizes a particular egg, or if the egg is fertilized at all. Eric Flint and his collaborators in the 1632 series take this approach. Even when taking this approach, what can happen is that parents make the same decisions about what to name the children they have. So, for example, both the original timeline and the 1632 timeline have a Baruch Spinoza who was conceived in early 1632, but genetically, they are siblings, and the chance of them being genetically identical is very small. On average, "alternate siblings" such as the Spinozas are no more or less likely to resemble one another than any other pair of full siblings.