noun /ˈælɡəɹɪðm/
A precise step-by-step plan for a computational procedure that begins with an input value and yields an output value in a finite number of steps.
Informally, an algorithm is any well-defined computational procedure that takes some value, or set of values, as input and produces some value, or set of values, as output. An algorithm is thus a sequence of computational steps that transform the input into the output.
Wikipedia foundation.