Recursion
![A visual form of recursion known as the Droste effect. The woman in this image holds an object that contains a smaller image of her holding an identical object, which in turn contains a smaller image of herself holding an identical object, and so forth. Advertisement for Droste cocoa, c. 1900](/Images/godic/202502/06/Droste5910.jpg")
![Ouroboros, an ancient symbol depicting a serpent or dragon eating its own tail.](/Images/godic/202502/06/Serpiente_alquimica5910.jpg")
![Recently refreshed sourdough, bubbling through fermentation: the recipe calls for some sourdough left over from the last time the same recipe was made.](/Images/godic/202502/06/Sourdough5910.jpg")
![The Sierpinski triangle—a confined recursion of triangles that form a fractal](/Images/godic/202502/06/Sierpinski_triangle.svg5910.png")
Recursion is the process of repeating items in a self-similar way. For instance, when the surfaces of two mirrors are exactly parallel with each other, the nested images that occur are a form of infinite recursion. The term has a variety of meanings specific to a variety of disciplines ranging from linguistics to logic. The most common application of recursion is in mathematics and computer science, in which it refers to a method of defining functions in which the function being defined is applied within its own definition. Specifically, this defines an infinite number of instances (function values), using a finite expression that for some instances may refer to other instances, but in such a way that no loop or infinite chain of references can occur. The term is also used more generally to describe a process of repeating objects in a self-similar way.