A deterministic algorithm is an Algorithm in which for a given input, the computer will always produce the same output by going through the same states that also means for given the same input, it will take the same amount of time/memory/resources every time it run.

In other words, I would say A deterministic algorithm is simply an algorithm that has a predefined output. For instance, if you are sorting elements that are strictly ordered (no equal elements) the output is well defined and so the algorithm is deterministic.

pseudo code:
function isNumberEven(n):
    if n % 2 = 0
    then return true
    else return false