Random.org is a website that produces "true" random numbers based on atmospheric noise.

In addition to generating random numbers in a specified range and subject to a specified probability distribution, it has free tools to simulate events such as flipping coins, shuffling cards, and rolling dice.

It also offers paid services to generate longer sequences of random numbers and act as a third-party arbiter for raffles, sweepstakes, and promotions.

Atmospheric noise is radio noise caused by natural atmospheric processes, primarily lightning discharges in thunderstorms.

On a worldwide scale, there are about 40 lightning flashes per second – ≈3.5 million lightning discharges per day.

A pseudorandom process is a process that appears to be random but is not.

Pseudorandom sequences typically exhibit statistical randomness while being generated by an entirely deterministic causal process.

Such a process is easier to produce than a genuinely random one, and has the benefit that it can be used again and again to produce exactly the same numbers - useful for testing and fixing software.

Randomness is the lack of pattern or predictability in events.

A random sequence of events, symbols or steps has no and does not follow an intelligible pattern or combination.

Individual random events are by definition unpredictable, but in many cases the frequency of different outcomes over a large number of events (or "trials") is predictable.

For example, when throwing two dice, the outcome of any particular roll is unpredictable, but a sum of 7 will occur twice as often as 4.

In this view, randomness is a measure of uncertainty of an outcome, rather than haphazardness, and applies to concepts of chance, probability, and information entropy.

Random number generation is the generation of a sequence of numbers or symbols that cannot be reasonably predicted better than by a random chance, usually through a random-number generator (RNG).