An order of magnitude is the class of scale or magnitude of any amount, where each class contains values of a fixed ratio to the class preceding it. In its most common usage, the amount being scaled is 10 and the scale is the (base 10) exponent being applied to this amount (therefore, to be an order of magnitude greater is to be 10 times as large). Such differences in order of magnitude can be measured on the logarithmic scale in "decades" (i.e. factors of ten).

It is common among scientists and technologists to say that a parameter whose value is not accurately known, or is within a range, is "of the order of" some value. For example, standby electrical power used in a household is not accurately known and varies between households, but is typically of the order of a few tens of watts.

Orders of magnitude are generally used to make very approximate comparisons, and reflect very large differences. If two numbers differ by one order of magnitude, one is about ten times larger than the other. If they differ by two orders of magnitude, they differ by a factor of about 100. Two numbers of the same order of magnitude have roughly the same scale: the larger value is less than ten times the smaller value. This is the reasoning behind significant figures: the amount rounded by is usually a few orders of magnitude less than the total, and therefore insignificant.

In mathematics, big O notation is used to describe the limiting behavior of a function when the argument tends towards a particular value or infinity, usually in terms of simpler functions. It is a member of a larger family of notations that is called Landau notation, Bachmann–Landau notation (after Edmund Landau and Paul Bachmann), or asymptotic notation. In computer science, big O notation is used to classify algorithms by how they respond (e.g., in their processing time or working space requirements) to changes in input size.

Big O notation characterizes functions according to their growth rates: different functions with the same growth rate may be represented using the same O notation. A description of a function in terms of big O notation usually only provides an upper bound on the growth rate of the function. Associated with big O notation are several related notations, using the symbols o, Ω, ω, and Θ, to describe other kinds of bounds on asymptotic growth rates.

Big O notation is also used in many other fields to provide similar estimates.

Seongha Jeong (정성하) (colloquially: Sungha Jung) (born 2 September 1996) is a South Korean professional acoustic finger-style guitarist who has risen to fame on YouTube and other sites. As of 2012, his channel had over 17 million views, with his videos getting a total of over 454 million views, and also over 726,000 subscribers.

Seongha typically takes three days to learn and practice a new piece, and video-record it for upload onto YouTube. His genre selection is rather broad, as he learns and plays many pieces that are playable on guitar, therefore consequently spread across numerous genres.

Seongha has won 13 awards on YouTube, including 6 "#1" awards. Also on YouTube, Seongha has 523 videos with over one million views. Seongha's video with the most views is the one which shows him playing the theme from "Pirates Of The Caribbean".

In 2010, he was featured on Narsha's solo album "Narsha" for the song "I'm in Love" Seongha has composed 18 pieces as of February 2011, two of which are featured in his debut album, "Perfect Blue". He released his second album, "Irony", on 21 September 2011.