In mathematics, the concept of idempotence implies that an operation can be executed any number of times and still yield the same result as if it was only run once. f(f(x)) = f(x).
Programming-wise, the word means that some method or function which mutates a variable has no additional effect on the variable if it is called on it more than once, given the same input parameters. For instance, if you initialize a singleton, then try to reinitialize a new instance of that singleton, you will recieve the same instance.
It’s fun to see the similar vocablary sping up in programming since I came from a theory-heavy math/stats/physics background with less programming. Excited to see more of this!