Big O notation - Wikipedia, the free encyclopedia In computer science, big O notation is used to classify algorithms by how they respond to changes in input size, such as how the processing time of an algorithm changes as the problem size becomes extremely large.[3] In analytic number theory it is used to estimate the "error committed" while replacing the asymptotic size of an arithmetical function by the value it takes at a large finite argument. A famous example is the problem of estimating the remainder term in the prime number theorem. in Software Engineeringwith algorithmalgorithmsclassifycomputernotationprocessingsciencetheory