A PRNG (pseudorandom number generator) is an algorithm that outputs numbers in a complex, seemingly unpredictable pattern. Truly random numbers (say, from a radioactive source) are utterly unpredictable, whereas all algorithms are predictable, and a PRNG returns the same numbers when passed the same starting parameters or seed.
Depending on quality of algorithm and entropy of seeding, PRNGs vary in security levels and therefore in application.
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General knowledge
- Pseudorandom number generator on Wikipedia
Math.random(), a built-in JavaScript PRNG function