In computer science, the time complexity of an algorithm is a function, which quantitatively describes the running time of the algorithm. This is a function of the length of the string representing the input value of the algorithm. Time complexity is often represented by a big O symbol, excluding the low-order term and the first term coefficient of the function.

The big O, which stands for "the order of", was originally a capitalized Greek letter' ο' (Omicron), and now it is a capitalized Latin letter' o'.