One of a sequence of numbers generated by some algorithm so as to have an even distribution over some range of values and minimal correlation between successive values.
Pseudorandom numbers are used in simulation and encryption. They are pseudorandom not random because the sequence eventually repeats exactly and is entirely determined by the initial conditions.
One of the simplest algorithms is
x[i+1] = (a * x[i] + c) mod m
but this repeats after at most m numbers and successive numbers are closely related. Better algorithms generally use more previous numbers to calculate the next number.